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FIELD OF THE INVENTION
The present invention relates to a disc type magnetic recording medium for high density recording comprising a magnetic layer and a non-magnetic layer and containing a ferromagnetic metal fine powder or hexagonal ferrite fine powder in the
uppermost layer.
BACKGROUND OF THE INVENTION
In the art of magnetic disc, it is a recent trend to use a 2 MB MF-2HD floppy disc made of Co-modified iron oxide as a standard with personal computers. However, the capacity of such a 2MB MF-2HD floppy disc is not always sufficient to meet the
recent rapid increase in the capacity of data to be processed. Thus, it has been desired to provide floppy discs having a higher capacity.
As magnetic recording media there have heretofore been widely used products obtained by coating on a non-magnetic support a magnetic layer having iron oxide, Co-modified iron oxide, CrO.sub.2, ferromagnetic metal powder or hexagonal ferrite
powder dispersed in a binder. Among these particulate materials, ferromagnetic metal fine powder and hexagonal ferrite fine powder are known to provide excellent high density recording properties.
Examples of large capacity discs using ferromagnetic metal fine powder having excellent high density recording properties include 10 MB MF-2TD and 21 MB MF-2SD floppy discs. Examples of large capacity discs using hexagonal ferrite fine powder
having excellent high density recording properties include 4 MB MF-2ED floppy disc and 21 MB floptical. However, these floppy discs leave something to be desired with respect to capacity and performance. In order to cope with these difficulties, many
attempts have been made to enhance the high density recording properties of these floppy discs.
On the other hand, a disc type magnetic recording medium comprising a thin magnetic layer and a functional non-magnetic layer has recently been developed. 100 MB class floppy discs have come into the market. In order to achieve the properties
of these floppy discs, some constitutions have been proposed. For example, JP-A-5-109061 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a constitution comprising a magnetic layer having Hc of not
less than 1,400 Oe and a thickness of not more than 0.5 .mu.m and a non-magnetic layer containing an electrically-conductive particle. JP-A-5-197946 discloses a constitution comprising an abrasive having a greater size than the thickness of the magnetic
layer. JP-A-5-290354 discloses a constitution having a magnetic layer with a thickness of not more them 0.5 .mu.m and a thickness fluctuate of .+-.15% and a predetermined surface electrical resistance. JP-A-6-68453 discloses a constitution containing
two abrasives having different particle diameters in a predetermined amount on the surface layer.
However, even these approaches can hardly provide satisfactory properties as the recording density of disc type magnetic recording media rapidly increases. The reason for this difficulty was investigated. As a result, it was found that as the
recording density of disc type magnetic recording media increases, the instability in the contact of the head with the media, including deterioration of so-called head contact and deformation of media, causes a remarkable deterioration of the magnetic
properties of the disc type magnetic recording media. It was also found that when the disc type magnetic recording medium is rotated at a high speed to enhance the transfer speed, it is entirely vibrated or vigorously fluttered, producing a spacing
between the head and the medium and hence making it impossible to obtain satisfactory properties.
In order to cope with various applications, it has been proposed to devise high density discs having various diameters. These disc media having different diameters are subject to the foregoing effects in different ways. It was thus found that
the solution to these problems is complicated.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a disc type magnetic recording medium which provides remarkable improvements in electromagnetic characteristics, particularly high density recording properties, and exhibits excellent magnetic
properties with various sizes.
The foregoing object of the present invention will become more apparent from the following detailed description and examples.
The inventors made extensive studies of the solution to the foregoing problems. As a result, it was found that the use of a polyethylene terephthalate as a support causes the foregoing difficulties in the conventional disc media. The use of a
support having a higher rigidity has been proposed. However, it was found that such a support has ill-balanced thickness and outer diameter and thus rather deteriorates the properties of the disc media than improves them. Further studies were made of
these problems. As a result, it was found that the foregoing object of the present invention can be accomplished by the following constitution. Thus, the present invention has been worked out.
The present invention relates to a disc type magnetic recording medium comprising a substantially non-magnetic layer and a magnetic layer provided on a non-magnetic support in this order, said magnetic layer comprising a ferromagnetic metal fine
powder or ferromagnetic hexagonal ferrite fine powder dispersed in a binder, wherein said non-magnetic support is an aramide or polyethylene naphthalate having a central plane average surface roughness SRa of not more than 10 mm and there is the
following relationship between the thickness d (.mu.m) and the outermost diameter D (mm) of the recording area:
1.25.ltoreq.D/d.ltoreq.2.50 (when the non-magnetic support is an aramide)
1.00.ltoreq.D/d.ltoreq.2.00 (when the non-magnetic support is a polyethylene naphthalate)
BRIEF DESCRIPTION OF THE DRAWING
By way of example and to make the description more clear, reference is made to the accompanying drawing in which:
FIG. 1 is a plan view illustrating an embodiment of the disc type magnetic recording medium according to the present invention, wherein a indicates the diameter of the central hole, b indicates the outer diameter of a disc, e indicates the
outermost periphery of the recording area, and D indicates the outermost diameter of the recording area.
DETAILED DESCRIPTION OF THE INVENTION
It was found that the disc type magnetic recording medium according to the present invention exhibits unprecedetedly excellent electromagnetic characteristics in all environments with all sizes.
In the present invention, there is the following relationship between the thickness d (.mu.m) of the non-magnetic support and the outermost diameter D (mm) of the recording area:
1.25.ltoreq.D/d.ltoreq.2.50, preferably 1.50.ltoreq.D/d.ltoreq.2.00 (when the non-magnetic support is an aramide)
1.00.ltoreq.D/d.ltoreq.2.00, preferably 1.30.ltoreq.D/d.ltoreq.1.70 (when the non-magnetic support is a polyethylene naphthalate)
Relationship between constitution and effect
The reason for the effect of the present invention is unknown but can be thought as follows. In the case where high density recording is effected on a disc type magnetic recording medium, it is necessary to properly and invariably control the
contact of the head with the medium. However, it is thought that when the rotary speed of the disc is increased, the following phenomenon occurs. When the disc is rotated at a high speed, the head can easily float. At the same time, the flexible disc
medium which rotates at a high speed shows deformation or fluctuation such as vibration and fluttering more vigorously. These phenomena make it difficult to stabilize the position of the head and the medium relative to each other, causing an output
drop. It was found that this trouble occurs most remarkably on the periphery of a disc which rotates at a high peripheral speed. It is presumed that the floating of the head depends not only on the peripheral speed of the disc but also on the surface
properties of the medium and the deformation or fluctuation depends on the rigidity of the medium.
Further, a flexible disc medium exhibits different deflectabilities at different points from the center thereof or from the edge of the central hole thereof when pressed under the head. Therefore, it is necessary that the rigidity of the disc be
controlled to stabilize the head contact. Accordingly, it is thought that the rigidity of the medium needs to be changed to obtain a proper deflectability depending on the size of the disc.
On the other hand, it is thought that the flexible disc medium deforms after the shape of the head around the head to keep its contact with the head constant. It is presumed that this phenomenon depends on the peripheral speed and the rigidity
and surface properties of the medium.
The foregoing phenomena are too complex to predict accurately.
The disc medium comprising as a support an aramide or polyethylene naphthalate (PEN) according to the present invention was experimentally confirmed to show a great fluctuation or deformation when subjected to high density recording at a high
rotary speed and far better properties than the disc medium comprising the conventional polyethylene terephthalate, i.e., prevent fluttering at the periphery thereof, where its contact with the head can hardly be stabilized due to the high peripheral
speed, improving its contact with the head.
It is presumed that the foregoing effect comes from the fact that the aramide or polyethylene naphthalate having a different rigidity from polyethylene terephthalate has a constitution as disclosed herein to make the head and the medium
well-balanced in the foregoing complex phenomena.
Description of support
The non-magnetic support to be used herein is made of an aramide or polyethylene naphthalate. The thickness d of the non-magnetic support satisfies the foregoing relationship. In the present invention, the thickness d indicates an average
thickness. In some detail, using a digital thickness meter ("MINICON" available from Tokyo Seimitsu Co., Ltd.), 10 or more samples are measured. The measurements are averaged to determine the average thickness d. The range of d is determined from the
relationship 0.4 D.ltoreq..ltoreq.d.ltoreq.0.8 D, which is derived from the relationship 1.25.ltoreq.D/d.ltoreq.2.50, when the non-magnetic support is made of an aramide. For example, when D is 35 mm, d is determined from the relationship 14
.mu.m.ltoreq.d.ltoreq.28 .mu.m. When D is 140 mm, d is determined from the relationship 56 .mu.m.ltoreq.d.ltoreq.112 .mu.m. On the other hand, when the non-magnetic support is made of a polyethylene naphthalate, the range of d is determined from the
relationship 0.5 D.ltoreq.d.ltoreq.1.0 D, which is derived from the relationship 1.00.ltoreq.D/d.ltoreq.2.00. For example, when D is 35 mm, d is determined from the relationship 17.5 .mu.m.ltoreq.d.ltoreq.35 .mu.m. When D is 140 mm, d is determined
from the relationship 70 .mu.m.ltoreq.d.ltoreq.140 .mu.m.
In the present invention, a lamination type support may be optionally used to make difference in surface roughness between the magnetic layer side of the disc and the other side (base side) of the disc. These supports may be previously subjected
to corona discharge treatment, plasma treatment, adhesive reception treatment, heat treatment, dust-proofing treatment, etc.
In order to accomplish the objects of the present invention, it is necessary that a material having a central plane average surface roughness (SRa) of not more than 10 nm, preferably not more than 5 nm, as determined by mirau method using TOPO-3D
(available from WYKO Inc.) be used as a non-magnetic support. Such a non-magnetic support preferably not only has a small central plane surface average surface roughness but also is free of protrusion having a size of not less than 0.5 .mu.m. The
surface roughness of the non-magnetic support can be freely controlled by properly selecting the size and amount of a filler to be optionally incorporated in the support. Examples of the filler used herein include oxide and carbonate of Ca, Si and Ti,
and finely divided powder of organic material such as acryl. The maximum height SRmax of the support is preferably not more than 1 .mu.m. The surface roughness averaged over 10 samples, SRz is preferably not more than 0.5 .mu.m. The central plane peak
height SRp is preferably not more than 0.5 .mu.m. The central plane valley depth SRv is preferably not more than 0.5 .mu.m. The central plane area factor SSr is preferably from 10% to 90%. The average wavelength S.lambda.a is preferably from 5 .mu.m
to 300 .mu.m. In order to obtain the desired electromagnetic characteristics and durability, the distribution of surface protrusion on the support may be properly controlled. Thus, the distribution of surface protrusion on the support can be controlled
such that surface protrusion having a size of from 0.01 .mu.m to 1 .mu.m are arranged in an amount of from 0 to 2,000 per 0.1 mm.sup.2.
When the non-magnetic support to be used in the present invention is made of an aramide, its F-5 value is preferably from 30 to 60 kg/mm.sup.2. The non-magnetic support preferably exhibits a thermal shrinkage factor of not more than 1%, more
preferably not more than 0.7% at 150.degree. C. for 30 minutes, or not more than 0.5%, more preferably not more than 0.1% at 105.degree. C. for 30 minutes. The non-magnetic support exhibits an elastic modulus of from 800 to 2,000 kg/mm.sup.2,
preferably from 1,000 to 1,500 kg/mm.sup.2. The non-magnetic support exhibits a thermal expansion coefficient of from 10.sup.-4 to 10.sup.-8 /.degree.C., preferably from 10.sup.-5 to 10.sup.-6 /.degree.C. The non-magnetic support exhibits a humid
expansion coefficient of not more than 10.sup.-4 /RH %, preferably not more than 10.sup.-5 /RH %. These thermal properties, dimensional properties and mechanical strength properties each preferably vary within 10% from the mean value in various
directions along the surface of the support.
On the other hand, when the non-magnetic support to be used in the present invention is made of a polyethylene naphthalate, its F-5 value is preferably from 10 to 40 kg/mm.sup.2. The non-magnetic support preferably exhibits a thermal shrinkage
factor of not more than 0.5%, more preferably not more than 0.3% at 105.degree. C. for 30 minutes, or not more than 0.3%, more preferably not more than 0.2% at 80.degree. C. for 30 minutes. The non-magnetic support exhibits an elastic modulus of from
500 to 1,400 kg/mm.sup.2, preferably from 600 to 1,000 kg/mm.sup.2. The non-magnetic support exhibits a thermal expansion coefficient of from 10.sup.-4 to 10.sup.-8 /.degree.C., preferably from 10.sup.-5 to 10.sup.-6 /.degree.C. The non-magnetic
support exhibits a humid expansion coefficient of not more than 10.sup.-4 /RH %, preferably not more than 10.sup.-5 /RH %. These thermal properties, dimensional properties and mechanical strength properties each preferably vary within 10% from the mean
value in various directions along the surface of the support.
In order to obtain an aramide support or polyethylene naphthalate support having the foregoing properties, it is preferred to properly control the longitudinal and crosswise orientation factor during film making so that no difference in
properties occurs along the surface of the disc medium.
Description of shape and layer structure
The shape of the disc magnetic recording medium of the present invention is not specifically limited. In the present invention, the outermost diameter D of the recording area must satisfy the foregoing relationship. FIG. 1 is a plan view
illustrating an embodiment of the disc magnetic recording medium according to the present invention wherein b represents the outer diameter of the disc and D corresponds to the outermost diameter of the recording area.
The value of D is not specifically limited. In a preferred embodiment, examples of D include from 70 mm to 140 mm for the purpose of large capacity and from 35 mm and less than 70 mm for the purpose of small size. The former value exerts its
effect more greatly on recording/reproduction at a peripheral speed of not less than 10 m/sec. at the outermost periphery e of the recording area. The latter value exerts its effect more greatly on recording/reproduction at a peripheral speed of not
less than 5 m/sec. at the outermost periphery e of the recording area.
There is the relationship V=.alpha..multidot..pi.D/60000=5.23.times.10.sup.-5 .alpha..multidot.D (m/sec.) (.alpha.=rpm) between the peripheral speed V of the outermost periphery of the recording area and the rotary speed .alpha. (rpm) of the
disc. Accordingly, V can be determined from D and the rotary speed .alpha..
The rotary speed .alpha. of the disc is normally from 600 rpm to 8,000 rpm, preferably 2,000 rpm to 5,000 rpm.
The diameter of the central hole a of the disc is normally from 10 mm to 50 mm.
The disc medium rotates at a lower peripheral speed and thus flutters less but exhibits a higher rigidity at the innermost portion of the recording area than the outermost portion of the recording area. Therefore, the diameter (a+2 d) of the
innermost portion of the recording area is predetermined to the optimum distance from the center of the disc or the edge of the central hole of the disc such that sufficient head penetration properties can be provided. The distance f between the
outermost periphery of the central hole and the innermost portion of the recording area is normally from 1 mm to 40 mm, preferably from 3 mm to 20 mm. If the distance f is too short, the resulting disc medium disadvantageously exhibits too high a
rigidity at the innermost portion of the recording area. On the contrary, if the distance f is too long, the recording area is disadvantageously too small.
The distance c between the outermost periphery of the disc medium and the outermost periphery of the recording area is normally from 0.2 to 10 mm, preferably from 0.5 to 3.0 mm.
Referring to the thickness constitution of the magnetic recording medium of the present invention, the non-magnetic support needs to have a predetermined ratio to the outermost diameter D of the recording area. An undercoating layer may be
provided between the non-magnetic support and the non-magnetic layer or magnetic layer to enhance the adhesivity of these layers. The thickness of the undercoating layer is from 0.01 to 2 .mu.m, preferably from 0.02 to 0.5 .mu.m. The present
application is intended for a double-sided magnetic disc medium having a non-magnetic layer and a magnetic layer provided on both sides of a support. However, these layers may be provided on only one side of the support. In this case, a back coat layer
may be provided on the side of the support opposite the side on which the non-magnetic layer and the magnetic layer are provided to achieve an effect such as antistat effect and correction of curl. The thickness of the back coat layer is from 0.1 to 4
.mu.m, preferably from 0.3 to 2.0 .mu.m. As the material constituting the undercoating layer and back coat layer there may be used any known materials.
The thickness of the magnetic layer in the magnetic medium of the present invention may be optimized by amount of the saturation magnetization of the head, the head gap length and the band of record signal but is normally from 0.01 .mu.m to 1.0
.mu.m, preferably from 0.05 .mu.m to 0.5 .mu.m, more preferably from 0.05 to 0.4 .mu.m. The magnetic layer may consist of two or more layers having magnetic properties. A known multi-layer magnetic layer constitution may be employed.
The thickness of the non-magnetic layer as the subbing layer in the magnetic medium of the present invention is normally from 0.2 .mu.m to 5.0 .mu.m, preferably from 0.5 .mu.m to 3.0 .mu.m, more preferably from 1.0 .mu.m to 2.5 .mu.m. The
subbing layer in the magnetic medium of the present invention can achieve its effect so far as it is substantially non-magnetic. For example, if the subbing layer contains a small amount of magnetic materials as impurities or intentionally, it can
achieve the effect of the present invention. Needless to say, this can be considered to be substantially the same constitution as that of the present application. The term "substantially non-magnetic" as used herein is meant to indicate that the
subbing layer exhibits a residual magnetic flux density of not more than 100 G, preferably zero, or a coercive force of not more than 100 Oe, preferably zero.
Description of ferromagnetic metal fine powder
As the ferromagnetic metal fine powder to be used in the magnetic layer of the present invention there may be preferably used a ferromagnetic alloy comprising .alpha.-Fe as a main component. Such a ferromagnetic metal fine powder may contain
atoms such as Al, Si, S, Sc, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr and B besides the predetermined atoms. In particular, at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni
and B is preferably incorporated besides .alpha.-Fe. More preferably, at least one of Co, Y and Al is incorporated. The content of Co is preferably from 0 to 40 atm-%, more preferably from 15 to 35 atm-%, particularly preferably from 20 to 35 atm-%,
based on Fe. The content of Y is preferably from 1.5 to 12 atm-%, more preferably from 3 to 10 atm-%, particularly preferably from 4 to 9 atm-%, based on Fe. The content of Al is preferably from 1.5 to 12 atm-%, more preferably from 3 to 10 atm-%,
particularly preferably from 4 to 9 atm-%, based on Fe. Such a ferromagnetic metal fine powder may be treated with the dispersant, lubricant, surface active agent or antistatic agent described later before dispersion.
The ferromagnetic metal fine powder may contain a small amount of an hydroxide or oxide. The ferromagnetic metal fine powder used herein can be obtained by any known preparation method. Examples of these preparation methods include a method
which comprises reducing metal oxide with a composite organic acid salt (mainly oxalate) and a reducing gas such as hydrogen; a method which comprises reducing iron oxide with a reducing gas such as hydrogen to obtain Fe or Fe--Co particles; a method
which comprises pyrolyzing a carbonylated metal compound; a method which comprises adding a reducing agent such as sodium borohydride, hypophosphite and hydrazine to an aqueous solution of a ferromagnetic metal so that it is reduced, and a method which
comprises evaporating a metal in an inert gas to obtain a metal fine powder. The ferromagnetic metal fine powder thus obtained may be subjected before use to any one of known gradual oxidation processes such as process which comprises dipping the
material in an organic solvent, and then drying the material, a process which comprises dipping the material in an organic solvent, blowing an oxygen-containing gas through the material to form an oxide film thereon, and then drying the material and
process which comprises forming an oxide film on the surface of the material free from an organic solvent while the partial pressure of oxygen gas and an inert gas are properly adjusted.
The ferromagnetic metal fine powder to be incorporated in the magnetic layer of the present invention exhibits a specific surface area of from 45 to 80 m.sup.2 /g, preferably from 50 to 70 m.sup.2 /g as determined by BET method. If the specific
surface area of the ferromagnetic metal fine powder falls below 45 m.sup.2 /g, greater noises are produced. On the contrary, if the specific surface area of the ferromagnetic metal fine powder exceeds 80 m.sup.2 /g, the desired surface properties can be
hardly obtained. The crystal size of the ferromagnetic metal fine powder to be incorporated in the magnetic layer of the present invention is from 80 to 350 .ANG., preferably from 100 to 250 .ANG., more preferably from 140 to 200 .ANG.. The major axis
length of the ferromagnetic metal fine powder is from 0.02 to 0.25 .mu.m, preferably from 0.05 to 0.15 .mu.m, more preferably from 0.06 to 0.1 .mu.m. The acicular ratio of the ferromagnetic metal fine powder is preferably from 3 to 15, more preferably
from 5 to 12. The ferromagnetic metal fine powder has .sigma.s of from 100 to 180 emu/g, preferably from 110 emu/g to 170 emu/g, more preferably from 125 to 160 emu/g. The coercive force of the ferromagnetic metal fine powder is from 1,400 Oe to 3,500
Oe, more preferably from 1,800 Oe to 3,000 Oe.
The water content of the ferromagnetic metal fine powder is preferably from 0.01 to 2%. The water content of the ferromagnetic metal fine powder is preferably optimized depending on the kind of the binder used. The pH value of the ferromagnetic
metal fine powder is preferably optimized depending on the combination with the binder used. The pH value of the ferromagnetic metal fine powder is normally from 4 to 12, preferably from 6 to 10. If necessary, the ferromagnetic metal fine powder may be
subjected to surface treatment with Al, Si, P or oxide thereof. The amount is from 0.1 to 10% based on the weight of the ferromagnetic metal fine powder. When the ferromagnetic metal fine powder is thus subjected to surface treatment, the adsorption of
a lubricant such as aliphatic acid can be reduced to not more than 100 mg/m.sup.2. The ferromagnetic metal fine powder occasionally contains soluble inorganic ions such as Na, Ca, Fe, Ni and Sr ions. It is preferred that the ferromagnetic metal fine
powder is essentially free of these inorganic ions. However, if the content of these inorganic ions is not more than 200 ppm, it rarely has adverse effects on the properties of the ferromagnetic metal fine powder. The ferromagnetic metal fine powder to
be used herein preferably has little voids. The void of the ferromagnetic metal fine powder of the present invention is preferably not more than 20 vol-%, more preferably not more than 5 vol-%. The ferromagnetic metal fine powder of the present
invention may be acicular, ellipsoidal or spindle-shaped so far as it satisfies the foregoing requirements for particle size. The ferromagnetic metal fine powder itself preferably has a small SFD, more preferably a SFD of not more than 0.8. It is
necessary that Hc distribution of the ferromagnetic metal fine powder be narrowed. If SFD of the ferromagnetic metal fine powder is not more than 0.8, it gives good electromagnetic characteristics and a high output. Further, it provides a sharp
magnetization inversion and less peak shift favorable for high density digital magnetic recording. In order to minimize Hc distribution of the ferromagnetic metal fine powder, the particle size distribution of goethite in the ferromagnetic metal fine
powder may be narrowed. Alternatively, the ferromagnetic metal fine powder may be prevented from being sintered.
Description of hexagonal ferrite fine powder
Examples of hexagonal ferrite to be incorporated in the uppermost layer of the present invention include substituted barium ferrite, substituted-strontium ferrite, substituted lead ferrite and substituted calcium ferrite. These ferrites may be
substituted by Co. Specific examples of these hexagonal ferrites include magnetoplumbite type barium ferrite, magnetoplumbite type strontium ferrite, magnetoplumbite type ferrite coated with spinnel, and magnetoplumbite type barium ferrite and strontium
ferrite partly having a spinnel phase. The hexagonal ferrite may contain any atoms such as Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge and Nb besides the
predetermined atoms. In general, a hexagonal ferrite containing elements such as Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn, Nb--Zn--Co, Sb--Zn--Co and Nb--Zn may be used. Some raw materials or preparation methods provide hexagonal ferrites containing
specific impurities.
The particle size of the hexagonal ferrite is from 10 to 200 nm, preferably from 20 to 100 nm as calculated in terms of diameter of hexagonal plate.
In the case of reproduction by a magnetic resistant head, it is necessary that the resulting noise be minimized. This requires the use of hexagonal ferrite having a tabular diameter of not more than 40 nm. If the diameter of hexagonal ferrite
is not more than 10 nm, the resulting thermal fluctuation makes it impossible to expect a stable magnetization. On the contrary, if the diameter of hexagonal ferrite is not less than 20 nm, great noises are produced. In any case, high density magnetic
recording cannot be effected. The tabular ratio (diameter/thickness) of the hexagonal ferrite is preferably from 1 to 15, preferably from 2 to 7. If the tabular ratio of the hexagonal ferrite falls below this range, the packing of the hexagonal ferrite
in the magnetic layer is advantageously great but a sufficient orientation cannot be obtained. On the contrary, if the tabular ratio of the hexagonal ferrite falls exceeds 15, the resulting stacking between particles causes noise increase. The
hexagonal ferrite having the foregoing range of particle size exhibits a specific surface area of from 10 to 200 m.sup.2 /g as determined by BET process. The specific surface area thus determined almost corresponds to the value calculated from the
diameter and thickness of the tablet. The crystal size of the hexagonal ferrite is normally from 50 to 450 .ANG., preferably from 100 to 350 .ANG.. It is normally preferred that the distribution of tabular diameter and tabular thickness of tabular
particle is as narrow as possible. The degree of this distribution can be hardly represented by numerical value. However, the comparison of this distribution can be made by random measurement of 500 particles on TEM photograph of particles. In many
cases, the distribution is not normal. When the distribution is represented by the standard deviation divided by the average size (.sigma./average size), it is from 0.1 to 2.0. In order to sharpen the particle size distribution, the particle production
reaction system may be uniformized as much as possible and the particles thus produced may be treated such that the distribution thereof is improved. For example, a method is known which comprises selectively dissolving ultrafine particles in an acid
solution. A magnetic powder having a measured coercive force of from 500 to 5,000 Oe can be prepared. The higher Hc is, the better is high density recording. However, Hc is limited by the performance of the recording head. Hc of the hexagonal ferrite
is normally from 800 Oe to 4,000 Oe, preferably from 1,500 Oe to 3,500 Oe. If the saturation magnetization of the head exceeds 1.4 tesla, Hc of the hexagonal ferrite is preferably not less than 2,000 Oe. Hc of the hexagonal ferrite can be controlled by
the particle size (tabular diameter and tabular thickness), the kind and amount of constituent elements, substitution site of elements, particle production reaction conditions, etc. The saturation magnetization of the hexagonal ferrite is from 40 emu/g
to 80 emu/g. The saturation magnetization .sigma.s is preferably high. However, the finer is the ferrite powder, the smaller is .sigma.s. It is known that magnetoplumbite ferrite may be complexed with spinnel ferrite to improve .sigma.s.
Alternatively, the kind and added amount of constituent elements may be properly selected. Further, W type hexagonal ferrite may be used. The magnetic powder may be treated with a substance suitable for the dispersant and binder used before dispersion. As the surface treatment there may be used an inorganic or organic compound. Representative examples of such a compound include oxide and hydroxide of Si, Al, P, etc., various silane coupling agents, and various titanium coupling agents. The amount to
be added is from 0.1 to 10% based on the amount of the magnetic powder. The pH value of the magnetic powder, also, is important for dispersion. The pH value of the magnetic powder may be optimized to a range of from 4 to 12 by the dispersant and binder
used. The pH value of the magnetic powder may be selected to a range of from 6 to 10 from the standpoint of chemical stability and preservability of the medium. The water content of the magnetic powder, also, has an effect on dispersion. The water
content of the magnetic powder can be optimized by the dispersant and binder used but is normally selected to a range of from 0.01 to 2.0%. Examples of the process for the preparation of hexagonal ferrite include (1) a glass crystallization process
which comprises mixing barium oxide, iron oxide, a metal oxide which substitutes iron, boron oxide as a glass-forming substance, etc. in such a proportion that the desired ferrite composition is obtained, melting the mixture, rapidly cooling the molten
material to obtain an amorphous material, re-heating the amorphous material, washing the material, and then crushing the material to obtain a powdered barium ferrite crystal, (2) a hydrothermal reaction process which comprises neutralizing a solution of
barium ferrite composition metal salt with an alkali, removing the by-products therefrom, heating the material at a temperature of not lower than 100.degree. C. in a liquid phase, washing the material, drying the material, and then crushing the material
to obtain a powdered barium ferrite crystal, and (3) a coprecipitation process which comprises neutralizing a solution of barium ferrite composition metal salt with an alkali, removing the by-products therefrom, drying the material, treating the material
at a temperature of not higher than 1,100.degree. C., and then crushing the material to obtain a barium ferrite crystal powder. In the present invention, the preparation process is not specifically limited.
Description of subbing layer (i.e. non-magnetic layer)
The details of the subbing layer (hereinafter occasionally referred to as "lower layer" or "non-magnetic layer"), if used, will be described hereinafter. The inorganic powder to be incorporated in the subbing layer (i.e., non-magnetic layer) is
a non-magnetic powder. Examples of such a non-magnetic powder include inorganic compounds such as metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide and metal sulfide. Examples of the inorganic compound used herein include
.alpha.-alumina having an alpha conversion of not less than 90%, .beta.-alumina, .gamma.-alumina, .theta.-alumina, silicon carbide, chromium oxide, cerium oxide, .alpha.-iron oxide, goethite, corundum, silicon nitride, titanium carbide, titanium oxide,
silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, and molybdenum disulfide. These inorganic compounds may be used, singly or in combination.
Particularly preferred among these inorganic compounds are titanium dioxide, zinc oxide, iron oxide, and barium sulfate from the standpoint of sharpness of particle size distribution and great variety of means of providing function. Even more preferred
among these inorganic compounds are titanium dioxide and .alpha.-iron oxide. The particle size of these non-magnetic powders is preferably from 0.005 to 2 .mu.m. If necessary, non-magnetic powders having different particle sizes may be combined.
Alternatively, a single non-magnetic powder having a broader particle diameter distribution may be used to obtain similar effects. It is particularly preferred that the particle size of the non-magnetic powder is from 0.01 to 0.2 .mu.m. In particular,
if the non-magnetic powder is a granular metal oxide, it preferably has an average particle diameter of not more than 0.08 .mu.m. If the non-magnetic powder is an acicular metal oxide, it preferably has a major axis length of not more than 0.3 .mu.m.
The non-magnetic powder has a tap density of from 0.05 to 2 g/ml, preferably from 0.2 to 1.5 g/ml. The non-magnetic powder has a water content of from 0.1 to 5% by weight, preferably from 0.2 to 3% by weight, more preferably from 0.3 to 1.5% by weight.
The pH value of the non-magnetic powder is preferably from 2 to 11, particularly from 5.5 to 10. The non-magnetic powder has a specific surface area of from 1 to 100 m.sup.2 /g, preferably from 5 to 80 m.sup.2 /g, more preferably from 10 to 70 m.sup.2
/g. The crystal size of the non-magnetic powder is preferably from 0.004 to 1 .mu.m, more preferably from 0.04 to 0.1 .mu.m. The oil absorption of the non-magnetic powder is from 5 to 100 ml/100 g, preferably from 10 to 80 ml/100 g, more preferably from
20 to 60 ml/100 g as determined by DBP (dibutyl phthalate). The non-magnetic powder has a specific gravity of from 1 to 12, preferably from 3 to 6. The non-magnetic powder may be acicular, spherical, polyhedral or tabular. The non-magnetic powder has
a Mohs' hardness of not less than 4, preferably not less than 10. The non-magnetic powder has SA (stearic acid) adsorption of from 1 to 20 .mu.mol/m.sup.2, preferably from 2 to 15 .mu.mol/m.sup.2, more preferably from 3 to 8 .mu.mol/m.sup.2. The pH
value of the non-magnetic powder is preferably from 3 to 6. The non-magnetic powder is preferably subjected to surface treatment with Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2, ZrO.sub.2, SnO.sub.2, Sb.sub.2 O.sub.3, ZnO or Y.sub.2 O.sub.3. The surface
treatment with Al.sub.2 O.sub.3, SiO.sub.2, TiO.sub.2 and ZrO.sub.2 is more preferred from the standpoint of dispersibility. The surface treatment with Al.sub.2 O.sub.3, SiO.sub.2 and ZrO.sub.2 is particularly preferred. These surface treatments may be
used, singly or in combination. Alternatively, a co-precipitated surface-treated layer may be used depending on the purpose. A process may be used which comprises treating the non-magnetic powder with alumina, and then treating the surface layer of the
non-magnetic powder with silica. A process may be used which comprises treating the non-magnetic powder with silica, and then treating the surface layer of the non-magnetic powder with alumina. The surface-treated layer of the non-magnetic powder may
be porous depending on the purpose but normally is preferably homogeneous and dense. Needless to say, the amount of the surface treatment should be optimized depending on the binder and dispersion conditions used.
Specific examples of the non-magnetic powder to be incorporated in the lower layer of the present invention include Nanotite (available from Showa Denko K.K.), HIT-100, ZA-G1 (available from Sumitomo Chemical Co., Ltd.), .alpha.-hematite DPN-250,
DPN-250BX, DPN-245, DPN-270BX, DBN-SA1, DBN-SA3 (available from Toda Kogyo Corp.), titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, .alpha.-hematite E270, E271, E300, E303 (available from Ishihara Sangyo Kaisha, Ltd.),
titanium oxide STT-4D, STT-30D, STT-30, STT-65C, .alpha.-hematite .alpha.-40 (available from Titan Kogyo K.K.), MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, MT-100F, MT-500HD (available from Teika K.K.), FINEX-25, BF-1, BF-10, BF-20, ST-M (available from
Sakai Chemical Industry Co., Ltd.), DEFIC-Y, DEFIC-R (available from Dowa Mining Co., Ltd.), AS2BM, TiO2P25 (available from Japan Aerosol Co., Ltd.), and 100A, 500A (available from Ube Industries, Ltd.). These non-magnetic powders may be calcined before
use. Particularly preferred non-magnetic powders are titanium dioxide and .alpha.-iron oxide.
The lower layer may comprise carbon black incorporated therein to achieve known effects. In other words, the surface electrical resistance Rs of the lower layer can be reduced. Further, the light transmittance of the lower layer can be reduced. Moreover, the desired micro Vickers hardness can be obtained. The incorporation of carbon black in the lower layer can also achieve an effect of storing a lubricant. Examples of the carbon black used herein include furnace black for rubber, thermal
black for rubber, carbon black for color, and acetylene black. The carbon black to be incorporated in the lower layer should be optimized with respect to the following properties depending on the desired effect. The combined use of different carbon
blacks can obtain better effects.
The carbon black to be incorporated in the lower layer has a specific surface area of 100 to 500 m.sup.2 /g, preferably 150 to 400 m.sup.2 /g, and an oil absorption of 20 to 400 ml/100 g, preferably 30 to 200 ml/100 g as determined with DBP. The
carbon black has an average particle diameter of 5 to 80 m.mu., preferably 10 to 50 m.mu., particularly 10 to 40 m.mu.. The carbon black preferably has a pH value of 2 to 10, a water content of 0.1 to 10% and a tap density of 0.1 to 1 g/ml. Specific
examples of the carbon black used in the present invention include BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72 (available from Cabot Corp.), #3050B, 3150B, 3250B, #3750B, #3950B, #950, #650B, #970B, #850B, MA-600, MA-230, #4000, #4010
(available from Mitsubishi Chemical Corp.), CONDUCTEX SC, RAVEN 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250 (available from Columbia Carbon Corp.), and Ketjen Black EC (available from Akzo Co., Ltd.). These carbon blacks may
be surface-treated with a dispersant, grafted with a resin or partially graphtized before use. These carbon blacks may be added to the magnetic coating solution in the form of dispersion in a binder. These carbon blacks may be used in an amount of not
more than 50% by weight based on the weight of the foregoing inorganic powder or not more than 40% by weight based on the total weight of the non-magnetic layer. These carbon blacks may be used, singly or in combination. For the details of the carbon
black used in the present invention, reference can be made to Handbook of Carbon Black, Carbon Black Kyokai.
Further, an organic powder may be incorporated in the lower layer depending on the purpose. For example, acrylstyrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment may be used. Further, polyolefin
resin powder, polyester resin powder, polyamide resin powder, polyimide resin powder, and polyfluoroethylene resin may be used. The preparation of these organic powders can be accomplished by the method as described in JP-A-62-18564 and JP-A-60-255827.
For the binder, lubricant, dispersant, additives and solvent to be incorporated in the lower layer and the method for dispersing these components, those used for the magnetic layer can be employed. In particular, for the amount and kind of the
binder, additives and dispersant, the known technique for the magnetic layer can be employed.
Description of binder
As the binder resin to be used in the present invention there can be used known thermoplastic resins, thermosetting resins, reactive resins or mixture thereof. As the thermoplastic resins there can be used those having a glass transition
temperature of -100.degree. C. to 150.degree. C., a number-average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000, and a polymerization degree of about 50 to 1,000.
Examples of such thermoplastic resins include polymers or copolymers containing as constituent units vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic acid ester, vinylidene chloride, acrylonitrile, methacrylic
acid, methacrylic acid ester, styrene, butadiene, ethylene, vinyl butyral, vinyl acetal, vinyl ether, etc., polyurethane resins, and various rubber resins. Examples of the above mentioned thermosetting resins or reactive resins include phenol resin,
epoxy resin, polyurethane hardening resin, urea resin, melamine resin, alkyd resin, acrylic reactive resin, formaldehyde resin, silicone resin, epoxy-polyamide resin, mixture of polyester resin and isocyanate prepolymer, mixture of polyester polyol and
polyisocyanate, and mixture of polyurethane and polyisocyanate. These resins are further described in "Plastic Handbook", Asakura Shoten. Further, known electron radiation curing resins can be incorporated in the various layers. Examples of these
resins and their preparation methods are further described in JP-A-62-256219. These resins can be used singly or in combination. Preferred examples of such a combination of resins include a combination of at least one selected from vinyl chloride
resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer and vinyl chloride-vinyl acetate-maleic anhydride copolymer with a polyurethane resin, and a combination thereof with polyisocyanate.
Examples of the structure of polyurethane resins which can be used in the present invention include known structures such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester
polycarbonate polyurethane and polycaprolactone polyurethane. Of all these binders, those in which at least one polar group selected from --COOM, --SO.sub.3 M, --OSO.sub.3 M, --P.dbd.O(OM).sub.2, --O--P.dbd.(OM).sub.2) (in which M represents a hydrogen
atom or alkaline metal salt group), --OH, --NR.sup.2, --N.sup.+ R.sup.3 (in which R is a hydrocarbon group), epoxy group, --SH, and --CN has been introduced by copolymerization or addition reaction may be optionally used to obtain better dispersibility
and durability. The amount of such a polar group is in the range of 10.sup.-1 to 10.sup.-8 mol/g, preferably 10.sup.-2 to 10.sup.-6 mol/g.
Specific examples of these binders to be used in the present invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC and PKFE (available from Union Carbide); MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF,
MPR-TS, MPR-TM and MPR-TAO (available from Nisshin Kagaku Kogyo K.K.); 1000W, DX80, DX81, DX82, DX83 and 100FD (available from The Electro Chemical Industrial Co., Ltd.); MR-104, MR-105, MR110, MR100, RM555, 400X-110A (available from Nippon Zeon Co.,
Ltd.);, Nippollan N2301, N2302 and N2304 (available from Nippon Polyurethane Co., Ltd.); T-5105, T-R3080 and T-5201, Barnok D-400 and D-210-80, and Crisvon 6109 and 7209 (available from Dainippon Ink And Chemicals, Incorporated); Vylon UR8200, UR8300,
UR8700, RV530, RV280 (available from Toyobo Co., Ltd.); Diphelamine 4020, 5020, 5100, 5300, 9020, 9022 and 7020 (available from Dainichi Seika K.K.); MX5004 (available from Mitsubishi Chemical Industries Ltd.); Sunprene SP-150 (available from Sanyo Kasei
K.K.); and Salan F310 and F210 (available from Asahi Chemical Industry Co., Ltd.).
The content of the binder to be contained in the magnetic layer and non-magnetic layer of the present invention is normally in the range of 5 to 50% by weight, preferably 10 to 30% by weight based on the weight of the non-magnetic powder
(excluding carbon black) or magnetic powder. If a vinyl chloride resin is to be used, its content is preferably in the r | | |
