Rigid magnetic recording disks lubricated with fluorinated telechelic polyether

Rigid magnetic recording disk is lubricated by a thin coating of a fluorinated telechelic polyether polymer having at least one polar group such as --CONHCH.sub.2 CH.sub.2 OH, --COOCH.sub.3, --CONHCH.sub.2 C.sub.6 H.sub.5, and --COCF.sub.3 . The coating is surprisingly resistant to removal by commonly used cleaning agents.

U.S. application Ser. No. 100,699, filed Dec. 18, 1979 (Chernega) claims a 
magnetic recording medium comprising a flexible backing and magnetizable 
layer coated with the same family of fluorinated polymer lubricants as 
those of the present invention. 
FIELD OF THE INVENTION 
This invention relates to the lubrication of rigid magnetic recording 
disks. 
BACKGROUND OF THE INVENTION 
Disk packs, each consisting of a stack of spaced-apart rigid magnetic 
recording disks, have long been in widespread use as memories for 
electronic computers. Users have become accustomed to cleaning the 
recording surfaces of the disks periodically with a cloth containing 
isopropyl alcohol. About five years ago, a new style of disk pack began to 
be marketed which is called a data module. In contrast to the earlier 
packs wherein the heads are normally out of contact with the disks, the 
heads of the data module are in contact with the disks until the disks 
approach operating speed. This contact requires that the disks be 
lubricated. It is believed that the most widely used lubricants for the 
disks are members of a class of highly fluorinated polyethers having the 
repeating unit 
EQU --CF.sub.2 --O-- 
or 
EQU --CF.sub.2 --CF.sub.2 --O-- 
or 
##STR1## 
for example, polyethers such as 
##STR2## 
where n is an integer providing a Ferranti-Shirley viscosity (hereinafter 
measured at 1640 sec.sup.-1) of about 75-1350 centipoises. Compare U.S. 
Pat. Nos. 3,778,308 (Roller) and 3,919,719 (Wright). Such polymers are 
available commercially from E. I. duPont de Nemours & Co. as "Krytox" 143 
fluorinated oils. A preferred member of this class has been "Krytox" 143 
AD which has a Ferranti-Shirley viscosity of 1322 centipoises and 
reportedly has a viscosity index (ASTM D2270) of 145 and an average 
molecular weight of about 7000. When these fluorinated polymers are used 
to lubricate rigid disks they tend to be removed if the disks are cleaned 
with isopropyl alcohol. 
Some manufacturers of data modules buy prelubricated disks which may become 
contaminated during shipment and handling and hence need to be cleaned. If 
such cleaning were to remove the lubricant, the data modules would soon 
fall. The recording layers of the disks, which typically are thin coatings 
of fine magnetizable particles in a thermoset epoxy resin matrix, would 
prematurely wear away where contacted by the heads, and the matrix resin 
would gradually transfer to the heads to interfere with their flying 
capability. 
The next generation of rigid disk equipment may employ heads which 
permanently remain in contact with the recording surface, in which case 
premature removal of the lubricant would be even more troublesome. 
OTHER PRIOR ART 
Highly fluorinated polyether prepolymers have been provided with polar 
groups which can react with other polyfunctional molecules to form 
polyurethanes, polyesters, polysiloxanes, polyamides or the like as 
disclosed in U.S. Pat. Nos. 3,810,874 and 4,085,137 (Mitsch et al.). The 
latter at column 11, lines 42-49 teaches that the same highly fluorinated 
prepolymers containing polar groups are also useful as lubricants, but 
without illustrating that use. 
U.S. Pat. No. 4,097,388 (Snyder et al.) uses the same fluorinated 
prepolymers, except with different terminal polar groups, as lubricants 
for engine oils, hydraulic fluids, and greases. 
THE PRESENT INVENTION 
The present invention concerns one or a stack of spaced-apart rigid 
magnetic recording disks, the recording surfaces of which are lubricated 
by a very thin continuous coating which provides lubrication equal to that 
provided by the previously preferred "Krytox" 143 AD but is not removed by 
commonly used cleaning agents such as isopropyl alcohol. The lubricating 
coating is based on highly fluorinated polymers that are chemically 
similar to "Krytox" 143 AD but differ by containing highly polar groups. 
That lubricating coating comprises a fluorinated telechelic polyether 
polymer having a backbone comprising --C.sub.a F.sub.2a --O-- units where 
a is an integer from 1 to 4, which backbone is terminated by at least one 
polar group such that .mu..sup.2 /MW is at least 19.times.10.sup.-4 
Debye.sup.2 -moles/g. "Telechelic" polymers have low molecular weight and 
known functional terminal groups. 
The dipole moment .mu. of the fluorinated telomer or telechelic polymer can 
be calculated from the Onsager Relation 
##EQU1## 
where k=Boltzmann's Constant 
T=Temperature in .degree.K. 
MW=Molecular Weight 
N.sub.o =Avogadro's Number 
d=Density 
.epsilon..sub.s =Dielectric constant measured at low frequencies, 
.epsilon..sub..infin. =Dielectric constant measured at high frequencies, 
where the approximation is made through Maxwell's Relations that 
EQU .epsilon..sub..infin. =.eta..sub.D.sup.2 +(5-10%).eta..sub.D.sup.2 
.congruent.1.075 .eta..sub.D.sup.2 
where .eta..sub.D is the refractive index of the material at the sodium D 
line at 20.degree. C. 
The fluorinated telechelic polyether polymer preferably has a number 
average molecular weight of at least 1000 and a Ferranti-Shirley viscosity 
(measured at 1640 sec.sup.-1 and 20.degree. C.) of at least 20 centipoise. 
The number average molecular weight of fluorinated telechelic polyether 
polymers having more than one terminal polar group per polymer molecule is 
preferably somewhat higher as compared to telechelic polymers having only 
one terminal polar group. For example, where there are two terminal polar 
groups per polymer molecule, the number average molecular weight 
preferably exceeds 1500. 
Useful polar groups for the fluorinated telechelic polyethers include 
EQU --CO.sub.2 R 
wherein R is alkyl of 1 to 6 carbon atoms, aryl or alkaryl of 6 to 10 
carbon atoms; 
##STR3## 
wherein each of R' and R" is hydrogen, alkyl of 1 to 6 carbon atoms, 
benzyl, or --R" OH where R" is an alkylene group of 2 to 6 carbon atoms; 
EQU --C.sub.b H.sub.2b OH; 
EQU --C.sub.b H.sub.2b NR'R"; 
##STR4## 
or 
EQU --C(OH).sub.2 C.sub.b F.sub.2b+1 
wherein b is an integer of 1 to 4. 
Strongly acidic polar groups such as carboxylic or sulfonic acid groups are 
less desirable because they are potentially corrosive. This may be 
minimized by conversion to salts. Preferably the pKa of the telechelic 
polyether polymer is at least 1.0. 
Preferred fluorinated telechelic polymers have the backbone --[CF.sub.2 
CF.sub.2 O].sub.m --[CF.sub.2 O].sub.n --, where m is an integer from 
about 6 to 32 and n is an integer from about 12 to 52, as disclosed in 
U.S. Pat. Nos. 3,810,874 and 4,085,137, or the backbone 
--CF(CF.sub.3)CF.sub.2 O--.sub.n or --CF.sub.2 CF(CF.sub.3)O--.sub.n, 
where n is an integer from about 6 to 20, as disclosed in U.S. Pat. Nos. 
3,250,808 and 3,699,145. 
Such fluorinated telechelic polyethers are readily produced to have number 
average molecular weights within the preferred range of 1000-5000 and 
Ferranti-Shirley viscosities (measured at 1640.sup.-1 sec) within the 
preferred range of 20-2000 centipose. The fluorinated telechelic polyether 
polymer may also contain --CF.sub.2 CF.sub.2 -- units which increase the 
separation between the ether oxygens and tend to make it necessary to 
avoid number average molecular weights toward the high end of the 
preferred range of 1000-5000 mentioned above. Otherwise, the viscosity of 
the fluorinated telechelic polyether polymer might be too high to provide 
good lubrication. 
Thin coatings of the fluorinated telechelic polyether polymers on rigid 
magnetic recording disks are highly resistant to removal by isopropyl 
alcohol, even to repeated vigorous scrubbing with saturated cloths. The 
lubricant may be applied from dilute solutions or dispersions by wiping or 
by any other technique such as spraying or dipping. After drying, the 
surface should be buffed with a dry, soft tissue to distribute the 
lubricant and remove any excess. The buffing changes the appearance from a 
slight haze to a shine. If a 5-.mu.l drop of n-decane after 10 seconds has 
a diameter of about 3 mm, this indicates that a continuous coating has 
been applied. In the absence of any lubricant, the diameter of the drop 
may be about 10-12 mm. 
Because it is not known how much of the lubricant has been removed by the 
buffing and because the coatings are thin, it is difficult to determine 
their thickness. Since a monomolecular layer of a fluorinated telechelic 
polyether polymer is believed to be on the order of 40 Angstroms in 
thickness, it is assumed that the coating should be at least that 
thickness. Electron scattering measurements suggest that thicknesses of 75 
to 250 Angstroms should provide preferred results. It is believed that an 
unduly thick coating of a fluorinated telechelic polyether polymer can 
cause undue drag.

EXAMPLE 1 
0.3 g of the highly fluorinated telechelic polymer polyether, 
##STR5## 
where m and n are each integers providing an average molecular weight of 
about 2000, was dissolved in 29.7 g of CClF.sub.2 CCl.sub.2 F ("Freon" 
113). The Ferranti-Shirley viscosity of this polymer was 1440 cps. While a 
14-inch (35-cm) rigid magnetic recording disk was being rotated at 3600 
rpm, a soft tissue saturated with the solution was wiped across its 
recording surface several times. Then after two minutes during which the 
solvent volatilized, the surface was buffed with a dry soft tissue to 
distribute the lubricant and remove any excess. 
The recording surface of the rigid disk comprised fine acicular 
magnetizable iron oxide particles in a thermoset epoxy resin binder, as 
did those of the disks of the examples below. 
To identical disks were applied other highly fluorinated polymers 
containing at least one highly polar group. The backbone chains of these 
polymers are hereinafter referred to by the following abbreviations (the 
first of these having been used in Example 1): 
##STR6## 
where m and n are each integers. These lubricated disks were cleaned with 
isopropyl alcohol as follows: 
With the disk spinning at 3600 rpm, a tissue soaked in isopropyl alcohol 
was pressed under full fingertip pressure against the disk for about 30 
seconds while moving the tissue across the recording surface several 
times. Such cleaning removed the widely used "Krytox" 143 AD lubricant 
mentioned above, as indicated by surface tension tests. However, every 
fluorinated telechelic polyether polymer was resistant to such cleaning as 
indicated by surface tension tests using n-decane. 
The effectiveness of the lubricating coatings of various fluorinated 
polymers was measured by the following tests. 
Post-Cleaning Slide Test 
After cleaning with isopropyl alcohol as indicated above, the disk is 
rotated at 120 rpm on a data module spindle, a speed at which an IBM 
3348-type recording head remains in contact with the recording surface. 
After 24 hours at ordinary room temperature, the disk and head are then 
examined with the naked eye, and failure is indicated if either there is a 
burnished mark on the recording surface or the head has accumulated a 
visible residue. 
Post-Cleaning Sliding Friction Test 
During the Post-Cleaning Slide Test, the friction between the head and the 
recording surface is monitored periodically, and failure is indicated if 
more than 4.5 grams of frictional force is applied against the head, 
either continuously or intermittently at any time during the 24-hour 
period. 
If in this test a value of more than 4.5 grams is observed initially, this 
may indicate that the coating of lubricant is too heavy, in which event 
the coating thickness may be reduced to an acceptable level by buffing 
with a dry soft tissue. 
__________________________________________________________________________ 
TEST RESULTS 
Viscosity in cps. 
Post-Cleaning 
Post-Cleaning 
Example 
Lubricant No. Ave. M/W 
(Ferranti-Shirley) 
Slide Test 
Sliding Friction 
__________________________________________________________________________ 
Test 
##STR7## 2000 1440 passed passed 
2 R.sub.f.sbsb.1(CO.sub.2 CH.sub.3).sub.2 
2000 35 passed passed 
3 R.sub.f.sbsb.1(CH.sub.2 OH).sub.2 
4000 195 passed failed 
4 
##STR8## 2100 730 passed passed 
5 
##STR9## 2000 275 passed passed 
6 R.sub.f.sbsb.1(CH.sub.2NH.sub.2).sub.2 
800 30 passed failed 
7 R.sub.f.sbsb.1(COOH).sub.2 
2000 71 passed passed 
8 
##STR10## 2600 125 passed passed 
9 R.sub.f.sbsb.3COOH 
3200 1130 passed failed 
10 R.sub.f.sbsb.3CO.sub.2 CH.sub.3 
3200 345 passed passed 
11 
##STR11## 3300 1785 passed failed 
12 
##STR12## 3300 2100 passed failed 
13 R.sub.f.sbsb.4(COOH).sub.2 
3800 * failed failed 
14 R.sub.f.sbsb.4(CH.sub.2 OH).sub.2 
3600 * failed failed 
15 R.sub.f.sbsb.4(CO.sub.2 CH.sub.3).sub.2 
3400 * failed failed 
16 R.sub.f.sbsb.5(CFClCF.sub.3).sub.2 
540 50 passed failed 
__________________________________________________________________________ 
*indicates too high to measure 
The fluorinated polymers of Examples 1-12 had the following values 
(.epsilon..sub.s being measured at 100 Hz except Example 1 at 10,000 Hz 
and Example 11 at 1000 Hz): 
______________________________________ 
.mu..sup.2 /MW 
Example 
m n d .epsilon..sub.s 
.epsilon..sub..infin. 
.times. 10.sup.4 
______________________________________ 
1 8 14 1.772 23.0 1.876 809 
2 8 14 1.721 3.70 1.815 91.8 
3 16 28 1.814 2.55 1.805 37.6 
4 8 14 NA NA NA NA 
5 8 14 NA NA NA NA 
6 3 5 NA NA NA NA 
7 8 14 NA NA NA NA 
8 -- 15 1.874 2.22 1.814 20.6 
9 -- 18 1.80 2.36 1.825 27.6 
10 -- 18 1.848 2.79 1.822 46.6 
11 -- 18 1.90 3.68 1.843 80.2 
12 -- 18 NA NA NA NA 
______________________________________ 
NA = not available 
It is not known why the coatings of the fluorinated telechelic polyether 
polymers of Examples 3, 6, 9, 11 and 12 failed the Post-Cleaning Sliding 
Friction Test. It is believed that Example 6 failed because the number 
average molecular weight of the telechelic polymer was only 800. 
Subsequent experiments suggest that some of the coatings may have been too 
thin. It is believed that after moderate experimentation, each of those 
telechelic polymers could have provided coatings that would pass the 
Post-Cleaning Sliding Friction Test. 
When Example 1 was repeated except that the magnetic recording disk had a 
binder-free recording surface of U.S. Pat. No. 3,973,072 (Anderson), both 
the Post-Cleaning Slide Test and the Post-Cleaning Sliding Friction Test 
were passed. Such a binder-free recording surface can be applied by 
electrodeposition, chemical reduction, electroless plating, vapor plating, 
evaporation or sputtering.