Lubricant and magnetic recording medium using the same

A fluorine-containing lubricant having a linear structure devoid of polar groups at the terminals of its molecule, and has at least two polar groups on the intermediate carbon chain of the molecule is disclosed. A magnetic recording medium employing the lubricant is also disclosed.

FIELD OF THE INVENTION 
This invention relates to a lubricant. It also relates to a magnetic 
recording medium using the same. 
BACKGROUND OF THE INVENTION 
The contact start-stop system (CSS system) is the leading type of 
start-stop system in hard disk devices using a flying type magnetic head. 
In the drive using "CSS system", a magnetic head for recording and 
reproduction contacts the surface of a disk at the time of the start and 
stop of the disk's rotation. The magnetic head slides on the surface of 
the disk while in contact with the surface of the disk. Because of this, 
the magnetic head and the disk are often damaged by abrasion, and if this 
becomes serious, head crash occurs. 
Protective films composed of carbon, predominantly amorphous carbon, or 
silicon dioxide has been proposed on magnetic layers to prevent this 
problem. 
The use of a liquid lubricant such as a perfluoroalkylpolyether coated on 
the protective film has also been proposed because the use of the 
protective film only is considered insufficient; see U.S. Pat. No. 
3,778,308. 
The distance between the magnetic layer and the magnetic head must be 
shortened to increase recording density. To achieve this, it is necessary 
to reduce both the thickness of the protective layer and the roughness of 
disk surface. For improved durability, a thicker lubricant layer is 
preferred. However, when this thickness is increased, it not only makes 
the disk unsuited for high-density recording, but also causes adsorption 
of the magnetic head to the surface of the disk. To prevent this, it has 
recently been proposed to reduce the thickness of the lubricant layer to 
only several tens of .ANG. which corresponds to a lubricant layer of only 
several molecules thick. 
However, an interaction between the magnetic or protective layer formed by 
a thin film forming method such as dry-process plating (e.g., sputtering) 
or wet-process plating and the lubricant molecule is generally weak. 
Because of this, when the lubricant layer is only a few molecules thick, 
the lubricant is likely to be spun off or dissipated by centrifugal force 
when the disk is rotated or by negative pressure when the magnetic head 
passes by. These factors also decrease the durability of the lubricating 
function. 
Methods in which a polar group is bonded to the terminal of a 
perfluoroalkylpolyether chain provide an increased interaction between the 
magnetic layer or the protective layer and the lubricant molecule have 
been proposed, thereby the durability of the lubricating function is 
improved without increasing the thickness of the lubricant layer as 
described in U.S. Pat. Nos. 4,267,238 and 4,268,556, and JP-B-60-10368 
(the term "JP-B" as used herein means an "examined Japanese patent 
publication"). 
However, the present inventor has found that these procedures still 
insufficient. 
Magnetic recording mediums are shocked and rubbed by the magnetic head and 
are subjected to very severe use conditions. It is unavoidable that the 
lubricating film coated on the surface of the protective layer is 
scattered or liable to be unevenly distributed on the outer periphery of 
the disk by a centrifugal force. To avoid these problems, it has been 
proposed to introduce a polar group into the terminal of the 
perfluoroalkylpolyether chain. However, it has been found that, when the 
polar group is introduced into the terminal, the interaction between the 
protective layer and the lubricant layer is still insufficient, and also 
the adsorption produced by the terminal polar group is apt to be detached. 
It has also been found that the adsorption produced by the terminal polar 
group is apt to be more easily detached with increasing in the length of 
the molecular chain when the molecular weight of the 
perfluoropolyoxyethylene group is increased. 
It has now been found that, when a lubricant having a structure devoid of 
polar groups at the terminals of its molecule, but with polar groups on 
the intermediate carbon chain, both lubricating function and durability 
can be further improved. 
SUMMARY OF THE INVENTION 
The present invention provides a lubricant composed of a 
fluorine-containing compound having a specific structure and a magnetic 
recording medium using the lubricant. 
Another object of the present invention is to provide a magnetic recording 
medium which is excellent in lubricating function as well as durability. 
The present invention is characterized by a fluorine-containing lubricant 
having a linear structure. The lubricant is devoid of polar groups at the 
terminals of its molecule, but has at least two polar groups on the 
intermediate carbon chain of the molecule. 
The present invention also includes a magnetic recording medium comprising 
a substrate having thereon a magnetic layer and a lubricant layer wherein 
the lubricant layer is the uppermost layer and comprises the lubricant as 
described above. 
DETAILED DESCRIPTION OF THE INVENTION 
It is preferred that the lubricant of the present invention has a linear 
structure of a repeating unit of --(C.sub.n F.sub.2n O).sub.m --wherein n 
is an integer of 1 to 3, and m is an integer, and that the polar groups 
present on the intermediate carbon chain of the molecule are selected from 
the group consisting of the residue of a carbonyl group-containing 
compound, the residue of an isocyanate group-containing compound and the 
residue of a hydroxyl group-containing compound. Further, it is preferable 
that the terminal group of the lubricant is --CF.sub.3, and the lubricant 
has a number average molecular weight of about 4,000 to abut 100,000. 
Examples of the residue of a carbonyl group-containing compound, the 
residue of an isocyanate group-containing compound and the residue of a 
hydroxyl group-containing compound include the followings. 
##STR1## 
More preferably, the lubricant is a compound represented by the following 
formula (I): 
EQU R.sub.1 --(Y--R.sub.2).sub.k --Y--R.sub.1 (I) 
wherein Y is selected from the group consisting of the residue of a 
carboxyl group-containing compound, the residue of an isocyanate 
group-containing compound and the residue of a hydroxyl group-containing 
compound, and all Y groups in formula (I) may be the same or different; 
R.sub.1 represents a perfluoropolyether chain comprising a repeating unit 
of (C.sub.a F.sub.2a O).sub.b wherein a is 1, 2 or 3, and b is an integer 
of at least 1, preferably from 1 to 60, more preferably at least 2, which 
is terminated by --CF.sub.3 and has a number average molecular weight of 
about 400 to 10,000, preferably 4,000 to 10,000, and all R.sub.1 groups in 
formula (I) may be the same or different; R.sub.2 represents a 
perfluoropolyether chain comprising a repeating unit of (C.sub.c F.sub.2c 
O).sub.c wherein c is 1, 2 or 3, and d is an integer of at least 1, 
preferably from 1 to 60, more preferably at least 2 and having a number 
average molecular weight of about 1,000 to 10,000, preferably 1,000 to 
4,000, and all R.sub.2 groups in formula (I) may be the same or different; 
and k is an integer of at least 1. 
In the above formula (I), the group represented by R.sub.1 may comprise 
either exclusively the same units or plural units different from each 
other. 
It is furthermore preferred that 2 to 50 Y groups, more preferably 2 to 6 Y 
groups, are contained in the lubricant of formula (I). 
This lubricant does not have any polar group at its terminals, but has at 
least two polar groups on the intermediate carbon chain of the molecule. 
Even when the length of the linear structure is increased, the physical 
adsorption force of the lubricating film to the surface of the inorganic 
material such as metal, ceramic or carbon is strong, and it is difficult 
to detach. Namely, the lubricant is scattered by centrifugal force only 
with difficulty, and the lubricating function remains over a long period 
of time. 
Due to strong adsorption force, the lubricant remains its lubricating 
function over a long period of time when allowed to exist on the surface 
of the magnetic recording medium even if the lubricant layer is very thin 
such as only a few molecules in thickness. As a consequence, the distance 
between the magnetic layer and the magnetic head can be shortened, which 
makes the magnetic recording medium suited for high density recording. 
On the other hand, increasing the length of the liner structure in 
conventional types lubricants where the polar groups are bonded to the 
terminals of the linear structure makes the lubricant lager more likely 
detached from the medium. If the detached polar group is adsorbed by the 
magnetic head, a head crash is liable to occur. In the present invention, 
the polar groups are positioned not at the terminals of the molecule, but 
on the intermediate carbon chain of the molecule. Because of this unique 
arrangement, even when the length of the linear structure is increased, 
the polar group is only detached with difficultly. Even if the polar group 
is detached from the medium, the probability of it being adsorbed by the 
magnetic head is low because the molecule is not terminated by polar 
groups and this avoids head crashes. 
Specific examples of the lubricants suitable for use according to the 
present invention are each any one of the followings. However, the 
lubricants suitable for use according to the present invention are not to 
be construed as limited by these examples. 
##STR2## 
(wherein in the above formulae (1) to (3), l, m and n are integers, and k 
is an integer of at least 1, provided that the number average molecular 
weight of the compound of each formula is 4,000 to 100,000); 
##STR3## 
(wherein in the above formulae ( 4 ) to ( 9 ), j, l, m and n are integers, 
and k is an integer of at least 1, provided that the number average 
molecular weight of the compound of each formula is 4,000 to 100,000); and 
##STR4## 
(wherein in the above formulae (10) to (14), l, m and n are integers, and 
k is an integer of at least 1, provided that the number average molecular 
weight of the compound of each formula is 4,000 to 100,000) 
The magnetic recording medium according to the present invention can be 
prepared in a conventional manner. The lubricant layer comprising the 
lubricant of the present invention can also be provided in a conventional 
manner such as dip coating. 
Examples of materials for the substrate include, for instance, Al--Mg 
alloys, glass, carbon and titanium. 
Examples of material for the magnetic layer include, for instance, 
Co--Cr--Ta, Co--Cr--Pt and Co--Ni--Pt alloys.

The present invention is now illustrated in greater detail by reference to 
the following examples which, however, are not to be construed as limiting 
the present invention in any way. 
EXAMPLE 1 
To a 500 ml three-necked flask equipped with a stirrer, a reflux condenser 
and a thermometer, there were added 200 g of perfluoroalkylpolyether 
having both ends terminated by OH groups (i.e., the compound represented 
by formula: HOCH.sub.2 CF.sub.2 O--(CF.sub.2 CF.sub.2 O).sub.n --(CF.sub.2 
O).sub.m --CF.sub.2 CH.sub.2 OH; n/m=1; number average molecular 
weight=2,000) and 200 ml of 1,1,2-trichloro-1,2,2-trifluoroethane as a 
solvent. The mixture was uniformly mixed. While the resulting solution was 
stirred at 30.degree. C., 55 g of diphenylmethane diisocyanate dissolved 
in 100 ml of 1,1,2-trichloro-2,2-trifluoroethane was added dropwise 
thereto over a period of 10 minutes using a dropping funnel, and the 
mixture was reacted for 5 hours. Subsequently, 20 g of perfluoropolyether 
having one end terminated by a OH group (i.e., the compound represented by 
formula: F.paren open-st.CF.sub.2 CF.sub.2 CF.sub.2 O.paren 
close-st.CF.sub.2 CF.sub.2 CH.sub.2 OH; l is an integer; number average 
molecular weight=2,000) dissolved in 50 ml of 
1,1,2-trichloro-2,2-trifluoroethane was added dropwise over a period of 10 
minutes using a dropping funnel, and the mixture was continuously reacted 
for 5 hours. 
After cooling, the reaction solvent was removed under reduced pressure. The 
residue was extracted with perfluorohexane. The extract was repeatedly 
washed, and perfluorohexane was evaporated to obtain an oily product. 
The IR spectrum analysis of the oily product showed that absorption due to 
OH group present in the vicinity of 3600 to 3000 cm.sup.-1 disappeared, 
and absorption due to NH group of urethane group in the vicinity of 1540 
cm.sup.-1 and absorption due to CO group in the vicinity of 1735 cm.sup.-1 
appeared. 
The molecular weight of this product was measured. The product had a number 
average molecular weight of 10,000. The product was considered to be a 
compound having the following structural formula: 
##STR5## 
wherein l, n and m are integers, and k is an integer of at least 1. 
The product was a fluorine-containing compound having a linear structure 
with no polar groups at the terminals of the molecule; the terminal group 
was --CF.sub.3 and the compound had at least two polar groups on the 
intermediate carbon chain of the molecule. 
A Ni-P-plated aluminum substrate was abraded and subjected to a texture 
treatment to obtain a substrate having a center line surface roughness Ra 
of 4 nm. A Cr layer of 50 nm and a CoCrTa (Co: 76%, Cr: 12%, Ta: 2%) 
magnetic layer of 40 nm were formed on the substrate in an Ar gas 
atmosphere by a DC magnetron sputtering process. Further, a carbon 
protective film of 20 nm in thickness was formed thereon. Subsequently, 
the lubricant prepared above, diluted with Fluorinert FC77 was uniformly 
coated on the carbon protective film of the magnetic disk by a dip coating 
method and then cured to form a lubricant layer. 
The thickness of the lubricant layer was measured by a high sensitive 
reflection method using Fourier transform infrared spectrophotometer 
1720.times.(manufactured by Perkin-Elmer). The thickness was 21 .ANG.. 
EXAMPLE 2 
To a 500 ml four-necked flask equipped with a stirrer, a reflux condenser, 
a Dien Stark tube and a thermometer, there were added 200 g of 
perfluoroalkylpolyether having both ends terminated by COOH groups (i.e, 
the compound represented by formula: HOOC-CH.sub.2 CF.sub.2 O--(CF.sub.2 
CF.sub.2 O).sub.n --(CF.sub.2 O).sub.m --CF.sub.2 CH.sub.2 --COOH; n/m=1; 
average molecular weight=2,000), 4 g of p-toluenesulfonic acid and 200 ml 
of perfluorodecalin as a solvent, and mixture was uniformly mixed. While 
the resulting solution was stirred at 80.degree. C., 48 g of bisphenol A 
dissolved in 100 ml of perfluorodecalin was added dropwise over a period 
of 10 minutes by using a dropping funnel, and a dehydration-condensation 
reaction was continued until water was no longer collected in the Dien 
Stark tube. Subsequently, 20 g of perfluoropolyether having one end 
terminated by COOH groups (i.e., the compound represented by formula: 
F.paren open-st.CF.sub.2 CF.sub.2 CF.sub.2 O.paren close-st.--CF.sub.2 
CF.sub.2 COOH; number average molecular weight=2,000) dissolved in 50 ml 
of perfluorodecalin was added dropwise over a period of 10 minutes using a 
dropping funnel, and a dehydration-condensation reaction was continued 
until dehydrated water was no longer collected. 
After cooling, the reaction solvent was removed under reduced pressure. The 
residue was extracted with perfluorohexane, and the extract was repeatedly 
washed. Subsequently, perfluorohexane was evaporated to obtain an oily 
product. 
The IR spectrum analysis of the oily product showed that absorption due to 
a carbonyl group in the vicinity of 1780 cm.sup.-1 was shifted to nearly 
1735 cm.sup.-1. 
The molecular weight of the product was measured. The product had a number 
average molecular weight of 8,000. The product was considered to be a 
compound having the following structural formula: 
##STR6## 
wherein l, n and m are integers, and k is an integer of at least 1. 
The product was a linear fluorine-containing compound with no polar groups 
at the terminals of the molecule; the terminal group was --CF.sub.3 and 
the compound had at least two polar groups on the intermediate carbon 
chain of the molecule. 
A Ni-P-plated aluminum substrate was abraded and subjected to a texture 
treatment to obtain a substrate having a center line surface roughness Ra 
of 4 nm. A Cr layer of 50 nm and a CoCrTa (Co: 76%, Cr: 12%, Ta: 2%) 
magnetic layer of 40 nm were formed on the substrate in an Ar gas 
atmosphere by a DC magnetron sputtering process. Further, a carbon 
protective layer of 20 nm in thickness was formed thereon. The lubricant 
prepared above, diluted with Fluorinert FC77 was uniformly coated on the 
carbon protective layer of the magnetic disk by a dip coating method, and 
cured to form a lubricant layer. 
The thickness of the lubricant layer was measured by a high sensitive 
reflection method using Fourier transform infrared spectrophotometer 
1720.times.(manufactured by Perkin-Elmer). The thickness was 20 .ANG.. 
Comparative Example 1 
A lubricant layer was formed in the same manner as in Example 1 except that 
Fomblin AM2001 (tradename of a product manufactured by Montecatini) was 
used in place of the lubricant used in Example 1 to obtain a magnetic 
disk. The thickness of the lubricant layer was measured by a high 
sensitive reflection method using Fourier transform infrared 
spectrophotometer 1720.times.(a product of Perkin-Elmer). 
The thickness was 21 .ANG.. 
Comparative Example 2 
A lubricant layer was formed in the same manner as in Example 1 except that 
Fomblin Y-25 (tradename of a product manufactured by Montecatini) was used 
in place of the lubricant used in Example 1 to obtain a magnetic disk. 
The thickness of the lubricant layer was measured by a high sensitive 
reflection method using Fourier transform infrared spectrophotometer 
1720.times.(a product of Perkin-Elmer). The thickness was 19 .ANG.. 
Comparative Example 3 
A lubricant layer was formed in the same manner as in Example 1 except that 
a fluorinated polyether polymer wherein both ends thereof being terminated 
by OH group (lubricant described in JP-B-60-10368) was used in place of 
the lubricant used in Example 1 to obtain a magnetic disk. 
The thickness of the lubricant layer was measured by a high sensitive 
reflection method using Fourier transform infrared spectrophotometer 
1720.times.(a product of Perkin-Elmer). The thickness was 20 .ANG.. 
The magnetic disks prepared above were subjected to CSS test (a thin film 
head comprising a sintered material of Al.sub.2 O.sub.3 and TiC, 
manufactured by Read Write (U.S.A.), is used to a slider material, and a 
test is carried out by a 15 sec-operation/ 15 sec-stop cycle under 
conditions of head load=9.5 gf, F.H.=0.15 .mu.m and 3600 rpm), and the CSS 
resistance characteristics were examined by disk frictional wear tester 
PT101 (manufactured by Pattytech). The results are shown in Table 1. 
TABLE 1 
______________________________________ 
The number 
of times of CSS 
Scratch 
______________________________________ 
Example 1 80,000 times No scratch of the disk 
was observed. 
Example 2 70,000 times No scratch of the disk 
was observed. 
Comparative 50,000 times Scratches of the disk 
Example 1 were observed. 
Comparative 10,000 times Scratches of the disk 
Example 2 were observed. 
Comparative 10,000 times Scratches of the disk 
Example 3 were observed. 
______________________________________ 
It can be seen from the above results that the magnetic recording mediums 
obtained by using the lubricants of the present invention are excellent in 
durability. 
The lubricant of the present invention does not have polar groups at the 
terminals of the molecule, but has at least two polar groups on the 
intermediate carbon chain of the molecule; Even when the length of the 
linear structure is increased, the physical adsorption force of the 
lubricant film to the surface of the film of an inorganic material such as 
metal, ceramic or carbon is strong and is loosened difficultly. Thus, even 
when centrifugal force is applied to the magnetic recording medium, the 
lubricant is difficultly scattered, and hence the lubricating function 
remains over a long period of time. 
When the lubricant is allowed to exist on the surface of the magnetic 
recording medium, the absorption of the lubricant to the surface of the 
magnetic recording medium is strong, and hence the lubricating function 
remains over a long period of time even when the lubricant layer is very 
thin such as one molecule to several molecules thick. 
Accordingly, the distance between the magnetic layer and the magnetic head 
can be shortened, which makes the magnetic recording medium suited for 
high-density recording. 
Further, the magnetic layer and the magnetic head are difficultly damaged, 
and hence the magnetic recording medium is excellent in reliability. 
While the present invention has been described in detail and with reference 
to specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and the scope of the present invention.