Method for preparing magnetic recording medium

A method for preparing a magnetic recording medium, which comprises applying a thermal treatment at a temperature of 100.degree. C. or higher on a polymeric substrate comprising a polybiphenyl type imide having a molecular structure shown below, and then forming a magnetic layer on said substrate, and magnetic recording medium prepared by said method. ##STR1##

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for preparing a flexible magnetic 
recording medium. 
2. Related Background Art 
Pursuit of higher densification in magnetic recording has brought about 
high coercive force, thinner film of magnetic layer, etc. in magnetic 
recording medium. However, vertical magnetic recording has been proposed 
as the result of a pursuit of higher densification according to the method 
different from this, and practical applications have been progressed in 
various fields. The constitution of a vertical magnetic recording medium 
having the electromagnetic converting characteristics most approximate to 
practical application known in the art is so called flexible magnetic 
disc, comprising a magnetic layer of CoCr alloy thin film formed on a 
flexible polymeric substrate. 
Such constitution had the drawback which was essentially inevitable during 
formation of the magnetic layer that the medium is subject to curved 
deformation, namely generation of curl, due to internal stress of thin 
film. When curl is generated, no good contact can be obtained between the 
magnetic head and the medium, thereby inconveniences occur such as 
lowering in reproduction output, lowering in S/N ratio or instability in 
running, etc. In realizing practical application, prevention of curl is 
extremely important. 
One of the methods practiced in the prior art for prevention of curl of 
flexible magnetic disc is formation of magnetic layers on both surfaces, 
and another method is formation of a magnetic layer only on one surface to 
utilize heat shrinkage of the substrate, or provision of a coated layer on 
the back surface to cancel curl, but formation of magnetic layers on both 
surfaces is more suitable for practical application in view of simplicity 
of the steps. 
As practical preparation steps, as shown schematically in FIG. 1, there has 
been generally known the method in which magnetic layers are formed on the 
polymeric substrate by continuous sputtering or continuous vapor 
deposition while a lengthy polymeric substrate 4 is moved continuously. 
Also, there has been used the system in which the substrate is moved as 
flat without use of the rotatory drum as shown in FIG. 2. Here, the 
rotatory drum 3 or the substrate holder 7 is adapted to be elevatable to 
desired temperature when forming a magnetic layer on the polymeric 
substrate for improvement of the magnetic characteristics of the CoCr 
magnetic layer. As the polymeric substrate, polyethyleneterephthalate, 
polyimide, polyamide, etc. have been known as representative ones. 
However, according to such method of the prior art in which magnetic layers 
are formed on both surfaces of the polymeric substrates by use of 
polyethyleneterephthalate, polyimide, polyamide, etc., and the magnetic 
characteristics are improved by heating treatment during formation, the 
medium obtained suffered from curls generated irregularly in the longer 
direction of the medium, namely including the portion where curls can be 
removed and the portion where curls are generated, whereby curls cannot be 
removed stably. For this reason, there has been the problem that the 
production yield is low. 
On the other side, a thermal treatment of a substrate before formation of a 
magnetic layer is proposed, for example, in Japanese Patent Laid-Open 
Application Nos. 69-139139, 60-121532, 60-129931, 60-133540 and 60-191434. 
However, conditions in a thermal treatment a polyimide substrate are not 
well known. 
The use of polyimide as a substrate of a magnetic recording medium is 
described, for example, in Japanese Patent Laid-Open Application Nos. 
60-138722 and 62-215631. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method for preparing a 
flexible magnetic recording medium with little generation of curl by 
improving the polymeric substrate as described above. 
The present invention prepares a flexible magnetic recording medium by use 
of a polybiphenyl type imide having the molecular structure shown below as 
the polymeric substrate, and by carrying out the thermal treatment of said 
substrate at a temperature of 100.degree. C. or higher before forming a 
magnetic layer on said substrate:

DETAILED DESCRIPTION OF THE INVENTION 
The magnetic recording medium of the present invention comprises a magnetic 
layer formed on a polybiphenyl type imide polymeric substrate. The 
polybiphenyl type imide to be used in the present invention has a 
molecular structure of the formula shown below: 
##STR3## 
The polybiphenyl type imide is formed by condensation of p-phenylenediamine 
(PPD) and biphenyltetracarboxylic dianhydride (BPDA), excellent in heat 
resistance, dimensional stability and surface characteristic, and its 
thermally stable properties are particularly effective in a flexible 
magnetic recording medium of which substrate is heated during formation of 
magnetic film. More specifically, the above polybiphenyl type imide 
substrate has no glass transition point even when elevated in temperature, 
but it will be only decomposed gradually at 500.degree. C. or higher, and 
has a small thermal expansion ratio of 3.times.10.sup.-5 to 
1.times.10.sup.-6 cm/cm/.degree.C., with a high Young's modulus of 800 to 
1200 kg/mm.sup.2, as preferable for a thin substrate. The average value of 
the polymerization degree is 500,000 or more. 
Generally speaking, physical properties of polymeric substrates are liable 
to have hysteresis to heating, and for this reason, strain is generated on 
the substrate by the thermal treatment during formation of magnetic layer 
to cause generation of curl. Accordingly, for removing curl, in addition 
to having thermally stable properties, the extent of hysteresis is also an 
important factor. 
In this respect, the polybiphenyl type imide of the above molecular 
structure, as different from polyethyleneterephthalate, polyimide, 
polyamide of the prior art, can exclude its hysteresis by carrying out 
heat treatment. FIG. 3 is the result of measurement of the stretching of 
the polybiphenyl type imide when the cycle of heating-cooling was repeated 
by TMA (thermomechanical analysis instrument). The present inventors 
prepared flexible discs by varying the thermal treatment temperature, and 
examined about generation of curl, and consequently found that generation 
of hysteresis can be inhibited by heat treatment at 100.degree. C. or 
higher to remove stably curl. 
On the other hand, also for not deteriorating the magnetic characteristics 
of the magnetic layer, it is desirable to subject the polybiphenyl type 
imide of the above molecular structure at 100.degree. C. or higher. For 
example, the polybiphenyl imide generates gases such as H.sub.2 O, etc. 
when used in vacuum, and if no thermal treatment of the substrate is 
effected before formation of the magnetic layer, gases will be generated 
from the substrate by heating during formation of the magnetic layer, 
thereby deteriorating the magnetic characteristics of the magnetic layer. 
For this reason, it is preferable to subject said substrate to thermal 
treatment before formation of the magnetic layer, thereby liberating as 
much gases as possible. The amount of the gases generated from the above 
polybiphenyl type imide is abruptly increased at 100.degree. C. or higher 
in vacuum, and therefore it is desirable to subject the substrate to 
thermal treatment at 100.degree. C. or higher before formation of the 
magnetic layer also for the purpose of preventing deterioration of the 
magnetic characteristics of the magnetic layer. 
As the method for thermal treatment, for example, by use of a preparation 
device of magnetic recording medium known in the art as shown in FIG. 1 or 
FIG. 2, the rotatory drum 3 or the substrate holder 7 is heated to 
100.degree. C. or higher without formation of the magnetic layer, and the 
above polybiphenyl type imide is conveyed along the rotatory drum 3 or the 
substrate holder 7 to effect thermal treatment. The rotatory drum 3 or the 
substrate holder 7 are in themselves adapted to be elevated to desired 
temperatures for improvement of the magnetic characteristics of the 
magnetic layer, and therefore no special heating device is required. 
Otherwise, for example, the method of heating by an IR ray heater can be 
also used for carrying oui thermal treatment of the above polybiphenyl 
type imide substrate. Also, the thermal treatment may be carried out 
either in air or in vacuum. The thermal treatment time can be suitably 
determined depending on the film thickness of the substrate, but may be 
appropriately from 5 minutes to 25 minutes, further appropriately 6 
minutes to 20 minutes. The substrate should have a thickness preferably in 
the range of 10 to 100 .mu.m. Further, the thermal treatment can be 
suitably conducted at the temperature of 300.degree. C. or lower, further 
250.degree. C. or lower. 
After the above thermal treatment, the preparation steps may be performed 
according to the methods known in the art. More specifically, subsequent 
to the heating treatment of the substrate, a magnetic layer is formed by a 
sputtering device or the vacuum vapor deposition method, and heat is 
applied for improvement of magnetic characteristics to form a magnetic 
recording medium. Since the substrate has been already subjected to 
thermal treatment at 100.degree. C. or higher, substantially no hysteresis 
phenomenon will occur by heating during formation of the magnetic layer. 
The magnetic layer is formed on one surface or both surfaces of the 
substrate, but it is preferable to form magnetic layers on both surfaces, 
because curl can be generated with further difficulty. The thickness of 
the magntic layer, in both cases of forming it on one surface and both 
surfaces of the substrate, may be preferably about 0.4 .mu.m. 
As the ferromagnetic material for forming the magnetic layer, Co-Cr is 
preferred, but either a material which forms a magnetic layer for vertical 
magnetic recording to be magnetized in the direction perpendicular to the 
substrate surface, or a magnetic material for interplanar magnetic 
recording to be magnetized in the interplanar direction of the substrate 
may be available. For example, there can be included magnetic materials 
comprising ferromagneic metals or alloys composed mainly of Fi, Co, Ni 
such as Co, Co Ni, Co-Ni-P, Fe-Co, Fe-Cr, Co-V, Co-Rh, etc., or oxides, 
nitrides such as Co-O, Fe N, etc. The formulation ratio of Co-Cr can be 
also suitably selected. 
Also, the magnetic layer, in addition to the embodiment in which it is 
directly formed on the substrate, may be provided through a high 
permeability film as represented by a non magnetic film such as of Al, Ti, 
Cr, Ge, SiO.sub.2, Al.sub.2 O.sub.3, etc., or amorphous film such as Fe-Ni 
alloy film, Co-Zr, Fe-P-C, Fe-Mn-Bi, Fe-Co-Si-B, etc. 
Further, if necessary, a protective layer or a lubricating layer may be 
formed on the magnetic layer, or a back coat may be formed on the 
substrate surface opposite to the magnetic layer. 
By punching out the magnetic recording medium having a magnetic layer 
formed on one surface or both surfaces into disc shapes, flexible magnetic 
discs free from curl generation can be obtained. 
EXAMPLE 1 
By means a confronted target system sputtering device as shown in FIG. 1, 
heat treatment and formation of a magnetic layer were performed. The 
polymeric substrate 4 consisted of a polybiphenyl type imide of the 
formula: 
##STR4## 
(n has an average value of 500,000 or more), having a thickness of 25 
.mu.m and a width of 80 mm. After the substrate was set in said sputtering 
device, the device was evacuated to vacuum and the rotatory drum 3 
(diameter 300 mm) was elevated in temperature. The thermal treatment was 
carried out at a reached pressure of 2.times.10.sup.-3 Pa or less at the 
respective temperatures of 100.degree. C. and 190.degree. C. The tension 
of the polymeric substrate 4 was 1.2 kg, and its conveying speed was 6 
cm/min. 
After the thermal treatment, the system was evacuated to a reached pressure 
of 5.times.10.sup.-4 Pa or less, and then a CoCr magnetic layer was formed 
on the substrate. The target 6 had dimensions of 4 inch.times.6 
inch.times.6 mm, the distance between the targets was 120 mm, the distance 
from the center between the targets to the rotatory drum surface was 120 
mm, and the target had a composition of Co 80 wt. %-Cr 10 wt. %. Film 
forming conditions were argon pressure of 0.2 pa, thrown power of 2.5 kW, 
film forming speed of 1000 A/min., rotatory drum temperature of 
190.degree. C., tension of polymeric substrate of 1.2 kg, and the CoCr 
magnetic layer thickness was 0.4 .mu.m. After a magnetic layer was thus 
formed on one surface, also on the opposite surface was formed a CoCr 
magnetic layer to a thickness of 0.4 .mu.m under the same conditions. In 
FIG. 1 and FIG. 2, 1 is a delivery roller, 2 is a wind-up roller and 5 is 
a mask for restricting the flying magnetic particles. 
From the magnetic recording medium thus prepared, 15 discs of 47 mm in 
diameter were punched out and subjected to measurement of curling. The 
curling amount is shown in terms of average value of the heights of the 
both ends in the direction most curved when a disc is placed on a flat 
surface as shown in FIG. 4, namely Kp=1/2 (h.sub.1 +h.sub.2). The 
respective measurement results when the thermal treatment temperature are 
100.degree. C. and 190.degree. C. are shown in Table 1. 
EXAMPLE 2 
The same polybiphenyl type imide as in Example 1 with a thickness of 25 
.mu.m and a width of 80 mm was subjected to the thermal treatment in air 
by means of the device shown in FIG. 1. The thermal treatment conditions 
were tension of 1.2 kg, rotatory drum temperature of 190.degree. C., 
conveying speed of 6 cm/min. After the heat treatment, 15 magnetic discs 
were prepared in the same manner as in Example 1, and for each magnetic 
disc was measured curling amount. The results are shown in Table 1. 
EXAMPLE 3 
Two kinds of polybiphenyl type imide (the same as in Example 1) with 
thicknesses of 15 .mu.m and 50 .mu.m, and width of 80 mm for both were 
subjected to thermal treatments under the conditions of tension of 1.2 kg 
and conveying speed of 6 cm/min. in the case of the polybiphenyl type 
imide with a thickness of 15 .mu.m, and under the conditions of tension of 
2.0 kg and conveying speed of 3 cm/min. in the case of the polybiphenyl 
type imide with a thickness of 50 .mu.m. The temperature of the rotatory 
drum and the reached pressure were in both polybiphenyl type imides 
190.degree. C. and 2.times.10.sup.-3 Pa or less, respectively. After the 
heat treatment, 15 magnetic discs were prepared for each of the two kinds 
of polybiphenyl type imides, and for each disc, curling amount was 
measured. The measurement results are shown in Table 1. 
COMATIVE EXAMPLE 1 
The same polybiphenyl type imide as in Example 1 with a thickness of 25 
.mu.m and a width of 80 mm was used without thermal treatment to form a 
magnetic layer under the same conditions as in Example 1, thus preparing a 
magnetic recording medium. Further, 15 magnetic discs were prepared in the 
same manner as in Example 1, and curling amount was measured for each 
magnetic disc. The measurement results are shown in Table 1. 
COMATIVE EXAMPLE 2 
A magnetic recording medium was prepared in the same manner as in Example 1 
except for changing the thermal treatment temperature to 70.degree. C. 
Further, 15 magnetic discs were prepared in the same manner as in Example 
1, and curling amount was measured for each magnetic disc. The measurement 
results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
K.sub.p (mm) 
Comparative 
Example example 
Disc No. 
Example 1 2 Example 3 1 2 
__________________________________________________________________________ 
No 1 0.7 0.7 0.8 0.4 0.3 4.9 
1.5 
No 2 0.4 0.4 0.5 0.7 0.8 3.8 
2.8 
No 3 0.8 0.3 0.3 0.4 0.6 3.3 
1.6 
No 4 0.5 0.8 0.9 0.3 0.3 1.8 
0.9 
No 5 0.9 0.5 0.6 0.8 0.4 0.5 
1.9 
No 6 0.4 0.3 0.3 0.5 0.4 1.2 
2.3 
No 7 0.6 0.9 0.4 0.3 0.3 3.0 
1.2 
No 8 0.3 0.3 0.7 0.9 0.8 4.2 
2.4 
No 9 0.7 0.3 0.4 0.6 0.5 2.5 
2.7 
No 10 0.4 0.8 0.3 0.3 0.3 2.6 
2.3 
No 11 0.3 0.5 0.8 0.4 0.9 1.1 
1.4 
No 12 0.8 0.4 0.5 0.7 0.6 0.7 
1.7 
No 13 0.3 0.6 0.4 0.4 0.3 1.4 
2.3 
No 14 0.6 0.3 0.6 0.8 0.7 4.2 
2.8 
No 15 0.4 0.7 0.3 0.1 0.4 2.0 
2.2 
Average 
0.5 0.5 0.5 0.5 0.5 2.5 
2.0 
Value of 
15 discs 
Film 25 25 25 15 50 25 25 
Thickness 
(.mu.m) 
Heat 100 190 190 190 190 -- 70 
Treatment 
in in in air 
in in in 
Temperature 
vacuum 
vacuum vacuum 
vacuum vacuum 
(.degree.C.) 
__________________________________________________________________________ 
As described above, by using a polybiphenyl type imide for the substrate, 
and subjecting said substrate to heat treatment at a temperature of 
100.degree. C. or higher before formation of a magnetic layer on one or 
both surfaces, curl can be removed to produce flexible magnetic discs at 
good yield. Also, the thermal treatment step can be easily practiced 
sufficiently by means of a heating device of the prior art without 
requiring any special device.