.theta.-alumina powder, preparation thereof and magnetic recording medium

Alumina powder having a .theta.-alumina content of at least 70% by weight and an average primary particle size at least 0.1 .mu.m and containing a tungsten compound in an amount of 0.1 to 5% by weight in terms of tungsten oxide based on alumina can be used as an abrasive for a magnetic recording medium having good properties.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to .theta.-alumina powder, a method for 
preparing the .theta.-alumina powder, and a magnetic recording medium 
comprising the .theta.-alumina powder as an abrasive. 
2. Description of Related Art 
Generally, a magnetic recording medium for an audio, video and computer has 
a layer made of a magnetic paint which is prepared by dispersing a 
magnetic material such as ferromagnetic metal oxides or metal powder in a 
binder and coated on a non-magnetic substrate. Since the magnetic 
recording medium is used with contacting while being contacted to a 
magnetic head, the magnetic layer should have a sufficient travel property 
and durability. 
When the durability is insufficient, particles dropping from the magnetic 
layer adhere around the magnetic head to prevent a smooth contact of the 
magnetic layer with the magnetic head. Accordingly, an electromagnetic 
conversion property is deteriorated, a drop out arises and a tape is 
significantly contaminated during still reproduction. When the travel is 
insufficient, friction of the tape varies to deteriorate the travel 
property. 
Hitherto, inorganic powder such as alumina, silicon carbide, chromium 
oxide, titanium oxide, silicon oxide and .alpha.-iron oxide has been added 
as an abrasive to the magnetic layer so as to improve the travel property 
and durability of the magnetic layer. 
Alumina which is often used as the abrasive is predominantly 
.alpha.-alumina. It is known that aluminum hydroxide such as gibbsite, 
bayerite and boehmite can be converted to intermediate alumina and then 
.alpha.-alumina by heating as follows: 
.chi..fwdarw..kappa..fwdarw..alpha., .gamma..fwdarw..delta..fwdarw..theta. 
.fwdarw..alpha., .eta..fwdarw..theta..fwdarw..alpha., 
.rho..fwdarw..eta..fwdarw..theta..fwdarw..alpha., pseudo 
.gamma..fwdarw..theta..fwdarw..alpha. alumina (cf. for example, 
"Electrochemistry" (Japan), Vol. 28, page 302 by Funaki and Simizu; On 
alumina hydrate and alumina, Table 1, Examples of thermal change of 
alumina hydrate ). 
It is also known that amorphous alumina can be converted through 
intermediate alumina such as .gamma., .beta. and .theta.-alumina to 
.alpha.-alumina by pyrolysis of an aluminum salt such as aluminum 
chloride, aluminum sulfate and aluminum nitrate (cf. for example, 
"Mineralogy Journal" ( Japan ), Vol. 19, No. 1, page 21 and 41). 
The conversion from intermediate alumina to .alpha.-alumina is an 
exothermic reaction, and the grain growth of .alpha.-alumina is faster 
than that of intermediate alumina. .alpha.-Alumina used as the abrasive 
for the magnetic recording medium has a high Mohs hardness of 9 to 
contribute the improved travel property and durability of the magnetic 
layer. However, it is impossible to prevent incorporation of the coarse 
particles prepared by the grain growth curing the phase transition into 
.alpha.-phase, so that the coarse particles deteriorate a tape surface 
smoothness, scratch the magnetic head during the travel, and prevent a 
smooth contact with the magnetic head to deteriorate the electromagnetic 
conversion property. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide alumina powder used as an 
abrasive which can give a magnetic recording medium having good tape 
surface smoothness, good travel durability, a good electromagnetic 
conversion property and decreased damage of a magnetic head. 
Another object of the present invention is provide a method for preparing 
said alumina powder. 
Further object of the present invention is provide a magnetic recording 
medium comprising said alumina powder. 
The present invention relates to .theta.-alumina powder having a 
.theta.-alumina content of at least 70% by weight and an average primary 
particle size of at least 0.1 .mu.m and containing a tungsten compound in 
an amount of 0.1 to 5% by weight in terms of tungsten oxide based on 
alumina. 
The present invention also relates to a method for preparing 
.theta.-alumina powder which comprises calcining, at a temperature of 
1100.degree. to 1300.degree. C., an aluminum compound containing tungsten 
or a tungsten compound in an amount of 0.1 to 5% by weight in terms of 
tungsten oxide based on alumina after calcination to obtain alumina powder 
having a .theta.-alumina content of at least 70% by weight and an average 
primary particle size of at least 0.1 .mu.m and containing a tungsten 
compound in an amount of 0.1 to 5% by weight in terms of tungsten oxide 
based on alumina. 
The present invention further relates to a magnetic recording medium 
comprising a magnetic layer containing a magnetic material, a binder and 
an abrasive on a non-magnetic substrate, the abrasive being 
.theta.-alumina powder having a .theta.-alumina content of at least 70% by 
weight and an average primary particle size of 0.1 to 0.5 .mu.m.

DETAILED DESCRIPTION OF THE INVENTION 
The alumina powder according to the present invention has an alumina 
crystalline phase at least 70%, preferably at least 80% of which is 
.theta.-phase and an average primary particle size of at least 0.1 .mu.m. 
The alumina powder having the crystalline .theta.-phase is known. 
Commercially produced .theta.-alumina powder has an average primary 
particle size of at most 0.03 .mu.m. When it is intended to obtain the 
powder having the size of more than 0.03 .mu.m by thermal grain growth, 
the powder is quickly converted to .alpha.-alumina. Accordingly, no 
alumina powder having the average primary particle size of at least 0.1 
.mu.m and substantially consisting of .theta.-alumina prepared by thermal 
grain growth has been known. 
When .theta.-alumina is used as the abrasive for the magnetic recording 
medium, a sufficient head cleaning effect is achieved since the hardness 
of .theta.-alumina is larger than the hardness of silicon oxide, titanium 
oxide and .alpha.-iron oxide conventionally used as the abrasive, and a 
scratch on the magnetic head is decreased due to lower hardness than 
.alpha.-alumina. 
The .theta.-alumina powder used as the abrasive has the average primary 
particle size of at least 0.1 .mu.m, preferably from 0.1 to 0.5 .mu.m, 
more preferably from 0.12 to 0.3 .mu.m. A method for preparing the 
.theta.-alumina powder is not specifically limited, and a specific example 
of the preparation is as follows: 
The .theta.-alumina can be prepared by calcining, at a temperature of from 
1100.degree. to 1300.degree. C. for a time of from 10 minutes to 48 hours, 
preferably at a temperature of from 1150.degree. to 1250.degree. C. for a 
time of from 30 minutes to 24 hours, an aluminum compound raw material 
containing a tungsten compound in an amount of from 0.1 to 5% by weight, 
preferably from 1 to 3% by weight in terms of tungsten oxide based on 
alumina after calcination. 
The crystalline state of the resultant alumina powder drastically varies 
depending on the calcination temperature and time as well as the aluminum 
compound raw material and the kind and amount of the tungsten compound, it 
is recommendable to determine the calcination conditions on the basis of a 
simple preliminary experiment after selecting the raw material. 
The .theta.-alumina powder having a large size (for example, more than 0.2 
.mu.m) can be easily obtained, when the calcination is performed in the 
presence of H.sub.2 O vapor, for example, in a gas-firing furnace. 
A method for incorporating tungsten or the tungsten compound in the 
aluminum compound raw material is not limited, and tungsten or the 
tungsten compound is substantially homogeneously dispersed in the aluminum 
compound raw material. For example, tungsten or the tungsten compound is 
added to a solution containing an aluminum halide such as aluminum 
chloride or an aluminum salt such as aluminum sulfate, aluminum nitrate, 
aluminum perchlorate and aluminum alum in an amount of from 0.1 to 5% by 
weight in terms of tungsten oxide based on alumina after calcination, 
intimately mixed and then neutralized, recrystallized or precipitated in 
the form of a carbonate salt by ammonium hydrogencarbonate and the like to 
prepare the aluminum compound, which is then calcined. 
In another method, tungsten or the tungsten compound is added to a solution 
containing an organic aluminum compound, for example, an aluminum alkoxide 
such as aluminum methoxide, aluminum ethoxide and aluminum isopropoxide, 
an alkylaluminum such as trimethylaluminum and triethylaluminum, an 
aluminum carboxylate salt or an aluminum dicarboxylate salt in an amount 
of from 0.1 to 5% by weight in terms of tungsten oxide based on alumina 
after calcination, intimately mixed, and subjected to hydrolysis to 
prepare the aluminum compound, which is then calcined. 
In a further method, tungsten or the tungsten compound is added to an 
aluminum-containing compound prepared by neutralizing, recrystallizing or 
precipitating (in the form of a carbonate salt by ammonium 
hydrogen-carbonate) only the above aluminum salt, or an 
aluminum-containing compound prepared by subjecting only the above organic 
aluminum compound to hydrolysis or pyrolysis in an amount of from 0.1 to 
5% by weight in terms of tungsten oxide based on aluminum after 
calcination, and mixed in a dry or wet state to prepare a mixture, which 
is then calcined. 
It is preferable to use aluminum hydroxide having an average primary 
particle size of at most 0.05 .mu.m and prepared by the hydrolysis of the 
organic aluminum compound. 
When the amount of tungsten or the tungsten compound is outside the above 
range, it is impossible to prepare the desired .theta.-alumina powder 
having a large average primary particle size. 
The tungsten compound used as the raw material is not limited insofar as it 
can be homogeneously dispersed in or mixed with the aluminum compound raw 
material. Specific examples of the tungsten compound are an ammonium 
tungstate salt such as ammonium metatungstate and ammonium paratungstate, 
a tungsten halide such as tungsten chloride and a tungsten oxyhalide such 
as tungsten oxychloride. 
The magnetic recording medium according to the present invention is 
characterized in that the .theta.-alumina powder having the average 
primary particle size of 0.1 to 0.5 .mu.m and the .theta.-alumina content 
of at least 70% by weight is used as the abrasive. 
When the average primary particle size observed by a TEM (transmission 
electron microscope) is smaller than 0.1 .mu.m, a reinforcement effect is 
low and travel durability is low. When it is larger than 0.5 .mu.m, 
magnetic properties are low and the magnetic head may be scratched to 
deteriorate an electromagnetic conversion property. When the 
.theta.-alumina content is smaller than 70% by weight (namely the 
.alpha.-alumina content is larger than 30% by weight), surface smoothness 
of the tape is low and the magnetic head may be scratched during the 
travel. 
In the magnetic recording medium according to the present invention, an 
addition amount of the alumina powder is generally from 1 to 20 parts by 
weight, preferably from 1 to 15 parts by weight per 100 parts by weight of 
the magnetic material. When the amount is larger than 20 parts by weight, 
the tape surface smoothness and the magnetic properties are low and the 
magnetic head may be scratched. When the amount is smaller than 1 part by 
weight, the reinforcement effect is insufficient and the travel durability 
is low. 
The magnetic material may be a conventional one. Specific examples of the 
magnetic material are oxide magnetic materials such as .gamma.-Fe.sub.2 
O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Co-coated 
.gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing .gamma.-Fe.sub.3 
O.sub.4, Co-coated .gamma.-Fe.sub.3 O.sub.4 and CrO.sub.2, metal magnetic 
powder materials based on Fe, Ni or Co such as Fe, Ni, Co, an Fe-Ni alloy, 
an Fe-Co alloy, an Fe-Ni-P alloy, an Fe-Ni-Co alloy, an Fe-Mn-Zn alloy, an 
Fe-Ni-Zn alloy, an Fe-Co-Ni-Cr alloy, an Fe-Co-Ni-P alloy, a Co-Ni alloy, 
a Co-P alloy and a Co-Cr alloy. 
A particle size (average primary particle size) of the magnetic material is 
not limited, but is preferably from about 0.05 to about 5 .mu.m. 
A additive for the metal magnetic material may be used as in the prior arts 
and may be an element such as Si, Cu, Zn, Al, P, Mn and Cr and a compound 
comprising said element. 
The magnetic material may contain hexagonal system ferrite such barium 
ferrite, iron nitride and iron carbide. 
The binder used for the magnetic layer according to the present invention 
is a conventional one used for the magnetic recording medium. The binder 
may be thermoplastic resins, thermosetting resins, reactive resins and 
radiation (such as an electron beam) curing resins. 
Specific examples of the binder are urethane resins, vinyl chloride resins, 
epoxy resins, urea resins, amide resins, silicone resins, polyester 
resins, phenol resins, vinyl resins, cellulose derivative resins and 
rubberic resins. The resins may be a homopolymer or copolymer and may be 
used individually or in mixture. 
Specific examples of the material for the non-magnetic substrate are 
plastics, for example, polyesters such as polyethylene terephthalate and 
polyethylene naphthalate, polyolefins such as polypropylene, cellulose 
derivatives such as cellulose triacetate and cellulose diacetate, 
polyvinyl chloride resins, polycarbonates, polyamides and polysulfones as 
well as metals such as copper and ceramics such as glass. 
A method for preparing the magnetic recording medium according to the 
present invention may be a conventional one and is not limited. The 
magnetic recording medium can be prepared by mixing and dispersing the 
magnetic material, the binder, the abrasive and optional additives in an 
organic solvent to prepare a magnetic paint, coating the magnetic paint on 
the non-magnetic substrate, drying and optionally thermally treating the 
paint and then conducting a thermal or surface treatment. The magnetic 
layer of the magnetic recording medium according to the present invention 
generally has a thickness of 0.5 to 20 .mu.m. 
In addition to the abrasive for the magnetic recording medium, the 
.theta.-alumina powder according to the present invention can be used also 
as an additive for a back coating of magnetic recording mediums, and as a 
filler for paints, PET films and various fibers. 
PREFERRED EMBODIMENTS OF THE INVENTION 
The present invention will be illustrated by the following Examples which 
do not limit the present invention. 
Properties are measured as follows: 
Squareness ratio: 
A squareness ratio is measured at a sweep magnetic field of 15 K Gauss by a 
vibratory flux meter (BHV-50 manufactured by Riken Denshi). 
Surface gloss: 
Total refrectance at an incident angle of 60.degree. and a reflective angle 
of 60.degree. in a longitudinal direction of a sample tape is measured by 
means of a standard gloss meter, and a relative value thereof is 
determined by setting a specular glossiness to 100 at an incident angle of 
60.degree. in a glass having a refractive index of 1.56 according to JIS Z 
8741. 
Still characteristic: 
Using a video deck modified for still measurement, 4 MHz signals are 
recorded in a sample tape. When the signals are reproduced in the still 
state at 25.degree. C. and 60% RH under a back tension of 40 g, a time 
required for decreasing a reproducing output to a half is measured. 
Head deposition and head abrasion amount: 
When a sample tape is traveled for 100 hours at 40.degree. C. and 80% RH 
using a video deck NV-G21 manufactured by Matsushita, a deposit state on a 
magnetic head is observed and an abrasion amount of the head is measured. 
Average primary particle size: 
Particle sizes are read from TEM (transmission electron microscope) and 
converted to a weight base to prepare a cumulative frequency distribution 
curve and then the average primary particle size is determined. 
BET specific surface area: 
Using a directly reading-type specific surface area meter manufactured by 
Quantachrome, a N.sub.2 gas is absorbed and desorbed in a N.sub.2 --He 
carrier gas and a N.sub.2 desorption amount is measured by a thermal 
conductivity detector to determine a specific surface area. 
.theta.-Alumina content: 
X-ray intensity of a sample is measured, and a diffraction intensity in a 
plane index of .theta.-alumina is compared with a diffraction intensity of 
a standard sample (a X-ray intensity of a 1:1 mixture of .theta.-alumina 
and .alpha.-alumina is regarded as 50) which is simultaneously measured to 
show the .theta.-alumina content in % through the intensity ratio. 
EXAMPLE 1 
On aluminum hydroxide having an average primary particle size of 0.01 .mu.m 
which was prepared by hydrolysis of aluminum isopropoxide, a solution of 
ammonium metatungstate was sprayed in an amount of 2% by weight in terms 
of tungsten oxide based on alumina after calcination. The sprayed material 
was homogeneously mixed, charged in a thermal resistant alumina crucible 
and then calcined in a gas firing furnace at 1230.degree. C. for 3 hours. 
After cooling, the resultant alumina powder had an average primary particle 
size of 0.2 .mu.m. .theta.-Alumina constituted 89% of a crystalline phase 
of the alumina powder according to a X-ray diffraction, the balance being 
.alpha.-alumina. 
Five parts by weight of the resultant alumina powder was used to prepare a 
magnetic paint having the following composition: 
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Co-coated .gamma.-iron oxide 
100 parts by weight 
Polyurethane resin 10 parts by weight 
Viny chloride/vinyl acetate copolymer 
10 parts by weight 
Carbon 2 parts by weight 
Lubricant (butyl stearate) 
1 part by weight 
Curing agent (polyisocyanate) 
2 parts by weight 
Solvent 
Methyl ethyl ketone 100 parts by weight 
Toluene 100 parts by weight 
Cyclohexanone 50 parts by weight 
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After the above composition was dispersed for 5 hours in a sand mill and 
filtered through a filter having an average pore size of 5 .mu.m, it was 
coated on a polyethylene terephthalate film having a thickness of 14 .mu.m 
and dried to form a dried magnetic layer having a thickness of 3 .mu.m. 
After calendering, the film was cured at 70.degree. C. for 24 hours. Then, 
the film was cut in a width of 1/2 inch to prepare a magnetic tape. 
The surface gloss of the magnetic tape, the still characteristic, the 
deposit amount on the head and the head abrasion amount were measured. The 
results are shown in Table 2. 
EXAMPLE 2 AND COMATIVE EXAMPLES 1 TO 6 
Magnetic tapes were prepared in the same manner as in Example 1 except that 
the tungsten oxide content and calcination conditions were changed to 
prepare alumina powder having properties shown in Table 1 which was then 
used (In Comparative Example 3, commercial alumina powder (Al.sub.2 
O.sub.3 -manufactured by Degussa (Germany)) was used). 
The surface gloss of the magnetic tape, the still characteristic, the 
deposit amount on the head and the head abrasion amount were measured. The 
results are shown in Table 2. 
EXAMPLE 3 
To a solution of ammonium alum in water, ammonium metatungstate was added 
in an amount of 4% by weight in terms of tungsten oxide based on alumina 
after calcination. The mixture was intimately mixed. Tungsten-containing 
ammonium alum was precipitated by recrystallization and then calcined in a 
gas firing furnace at 1230.degree. C. for 3 hours to prepare 
.theta.-alumina powder having properties shown in Table 1. The 
.theta.-alumina powder was used to prepare a magnetic tape in the same 
manner as in Example 1. 
The surface gloss of the magnetic tape, the still characteristic, the 
deposit amount on the head and the head abrasion amount were measured. The 
results are shown in Table 2. 
EXAMPLE 4 
To a solution of aluminum isopropoxide in isopropyl alcohol, tungsten 
methoxide was added in an amount of 1% by weight in terms of tungsten 
oxide based on alumina after calcination. The mixture was intimately 
mixed. Tungsten-containing aluminum hydroxide was produced by hydrolysis 
and then calcined in a gas firing furnace at 1170.degree. C. for 3 hours 
to prepare .theta.-alumina powder having properties shown in Table 1. The 
.theta.-alumina powder was used to prepare a magnetic tape in the same 
manner as in Example 1. 
The surface gloss of the magnetic tape, the still characteristic, the 
deposit amount on the head and the head abrasion amount were measure. The 
results are shown in Table 2. 
TABLE 1 
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Preparation Properties of 
conditions alumina powder 
Tungsten Calcination 
Main .THETA.-Alu- 
Average 
oxide condition crystal- 
mina primary 
Example 
content (tempera- line content 
particle 
No. (wt %) ture .times. hr.) 
phase (%) size (.mu.m) 
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1 2.0 1230.degree. C. .times. 3 
.THETA. &gt;&gt; .alpha. 
89 0.22 
2 2.0 1280.degree. C. .times. 3 
.THETA. &gt;&gt; .alpha. 
75 0.30 
3 4.0 1230.degree. C. .times. 3 
.THETA. &gt;&gt; .alpha. 
85 0.23 
4 1.0 1170.degree. C. .times. 3 
.THETA. &gt;&gt; .alpha. 
92 0.12 
Com. 1 2.0 1150.degree. C. .times. 3 
.THETA. 
100 0.01 
Com. 2 2.0 1170.degree. C. .times. 3 
.THETA. 
100 0.05 
Com. 3 Al.sub.2 O.sub.3 --C 
(trade name) 
.delta. 
0 0.01 
Com. 4 0 1280.degree. C. .times. 3 
.alpha. &gt;&gt; .THETA. 
10 0.42 
Com. 5 1.0 1250.degree. C. .times. 3 
.alpha. &gt;&gt; .THETA. 
10 0.21 
Com. 6 0 1100.degree. C. .times. 3 
.THETA. &gt;&gt; .alpha. 
80 0.02 
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TABLE 2 
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Properties of tape 
Still Head 
Surface Square- charac- abrasion 
Exam- gloss ness teristic 
Head amount 
ple No. 
(%) ratio (min.) 
deposition 
(.mu.m/100 hr) 
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1 150 0.82 &gt;90 Good 4 
2 145 0.80 &gt;90 Good 4 
3 148 0.81 &gt;90 Good 5 
4 147 0.80 &gt;90 Good 4 
Com. 1 
130 0.78 60 Bad 2 
Com. 2 
137 0.79 70 Bad 3 
Com. 3 
135 0.79 60 Bad 2 
Com. 4 
138 0.80 &gt;90 Good 10 
Com. 5 
138 0.81 &gt;90 Good 4 
Com. 6 
135 0.78 30 Bad 2 
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When the alumina powder according to the present invention is used as an 
abrasive for a magnetic recording medium, the magnetic recording medium 
has good durability, magnetic properties and surface smoothness of a 
magnetic layer, a good cleaning effect on a magnetic head and a good 
prevention effect for magnetic head abrasion.