Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGR UND OF THE INYENTION
The object of the invention is the use of fluid
perfluoropolyethers having a very high kinematic viscosity,
comprised within the range of from 8,000 cSt to 40,000 cSt
at 20C, and preferably of at least 10,000 cSt, as
lubricating oils, and as the basic components of perfluo-
rinated lubricating greases.
The use is well-known of fluid perfluoropoly-
ethers, of neutral type, as lubricating oils endowed with
high performance characteristics relating to the resistance
to the atmospheric and chemical agents in general, even at
high temperatures.
The products presently available from the market,
suitable for use as lubricants, are low- or medium viscosity
oils, with maximum viscosity values of the order of 2,000
cSt at 20C.
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THE PRESENT INVENTION
It has been surprisingly found now that the
perfluoropolyethers having such a high kinematic viscosity
as indicated above, show surprisingly improved
charact~ristics of use, as lubricants, than the lower
viscosity perfluoropolyethers used up to date.
The improvement in some specific properties is not
attributal~ simply to the high viscosity of the product, and
should be regarded as unforeseeable.
The perfluoropolyethers according to the
invention, having viscosity values comprised within the
above specified range, are constituted by linear chains of
oxyperfluoroalkylene units selected from the following
units:
-CF2CF20-, -CF20-, -CF2CF2CF20- .
The chain end groups are of perfluorinated neutral
type.
In particular, the products belonging to the
following classes are suitable:
.
1) R~FO(CF2cF2O)m(cF2o)n f
wherein Rf and R'f, equal to, or different from each
other, are either -CF3 or -C2F5, the perfluorooxy-
alkylene units being randomly distributed along the
chain, and indices _, n are such integers that the
viscosity (which is a function of the average molecular
weight) is within the indicated ranges. Products of
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this type are disclosed in U.S. patent No. 3,665,041
and are known on the market under the trade name
"Fomblin z(R),,;
5 2 ) R ' f(CF2CF2)pRf
wherein Rf and R'f, equal LO, or different from each
other, are either -CF3 or -C2F5 and index p is such an
integer that the viscosity falls within the above
indicated limits. Products of this type are disclosed
in U.S. patent No. 4,523,039;
3) R'fO(CF2CF2CF2O)SRf
wherein Rf and R'f, equal to, or different from each
3~ or -C2F5 or -C3F7, and index s is an
integer such that the viscosity is comprised within the
above indicated ranges. Products of this type are
disclosed in European patent No 0 148 482.
On considering the very high viscosity of the
products, and that such a viscosity, because of their linear
structure, varies to a considerably low degree with varying
temperature, as well as their very low volatility, the
products of the invention can be advantageously used as
lubricants for high temperatures.
It is known that a relationship exists which
allows the friction coefficient ~ to be expressed as a
function of Stribeck number ~N/P, wherein ~ = dynamic
viscosity N = revolution speed: P = applied load per
surface unit. Such a relationship, reported on a diagram
allows the areas to be evidenced in which the system works
under boundary lubrication, mixed lubrication and
,,
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elastohydrodynamic lubrication conditions; on this regard,
reference is made to "Lubricants", by D. Klamann (Verlag
Chemie 1984), pages 33-foll..
The perfluoropolyether according to the present
invention shows a surprising and unexpected improvement as
relates to the wear rate under "mixed" lubrication
conditions, as compared to the up to date commonly used
perfluoropolyethers, having a much lower viscosity. This
improvement cannot be simply explained by taking into
account the difference in viscosity; it could be thought as
being in some way connected with the molecular structure of
the product.
The equipment used for measuring the wear rate is
essentially constituted by a pin-on-rig system, consisting
of a carbon steel (c 15) pin resting, for a surface area of
12 mm2, on the cylindrical surface, revolving at a rate of
350 rpm, of a specimen of the same material, having an
average surface roughness of 0.15 microns, and a diameter of
~5 mm. The pin is loaded with a weight of 15 kg.
~ The procedure used in the test was as follows.
The specimens and the pins are cleaned in 1,1,2-
trichloro-2,2,1-trifluoroethane, let dry and weighed. The
specimen is assembled on the equipment in such a way that it
can dip inside a tray containing the lubricating oil. After
1,000 revolutions, the specimen is dr~wn from the equipment,
is cleaned in 1,1,2-trichloro-2,2,1-trifluoroethane, let dry
and weighed again, and is then assembled again on the
equipment. The measurement is repeated after 10,000 rpm.
Then the difference in weight is computed relatively to the
number of run metres (~W/L):
~ r r
.2 !3S5~
Test Time Z 1600 Z 25 Z 03
(Minutes)~W/L (~/m) ~W/L (~/m~~/L (g/m)
3 O.lxlO 5 3xlO 53.1xlO 5
lxlO 5 2xlO 52.5xlO 5
Z 1600 Perfluoropolyether belonging to Class (1), a
commercial product Fomblin(R) Z by Montedison, having
a kinematic viscosity of 16,000 cSt at 20C.
Z 25 Ditto, with a viscosity of 250 cSt -
Z 03 Ditto, with a viscosity of 30 cSt.
The difference in the values measured between the
investigated products evidences a lower wear rate in the
high-viscosity oil. Above all, the necessary times for wear
to be obtained are much higher for the highest-viscosity
oil.
The perfluoropolyethers of the invention are
particularly fit for the formulation of perfluorinated
greases endowed with enhanced characteristics as compared to
analogous products manufactured from lower-viscosity
perfluoropolyether oils. Because of the very low value of
high-temperature oil separation which was observed (even at
a much higher oil percentage in the grease, than in the
products of the prior art), the novel grease type results
particularly suitable for high temperature usesO
A very meaningful characterizing property of the
new grease type formulated with the perfluoropolyether oil
according to the invention is the low static friction
coefficient (measured as the starting resistent torgue).
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The method of preparation of the greases according
to the invention is similar to the method as reported in
European patent application No. EP 0 095 825, with the
difference that, because of the high viscosity of the
perflu~ropolyether, the kneading and the following
homogenizing have to be performed under high temperature
conditions (70C).
The oil amounts used for formulating
polytetrafluoroethylene (PTFE)-thickened greases vary within
the range of from 60% to 90%.
EXAMPLE OF PREPARATION OF A PERFLUORINATED GREASE
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15Oil used is Fomblin(R) Z 1600, an above-mentioned
product, having a kinematic viscosity of 16,000 cSt at 20C.
It is introduced in the percentage of 74.1% by
weight, relatively to the grease.
The thickening agent is a granular PTFE (average
particle diameter 4-5 ~m; m.w. 40,000), used in the
percentage of 25.9% by weight, relatively to the grease.
25The kneading and homogenizing method is reported
in the above-cited patent.
To comparative purposes, a perfluorinated grease
was prepared by using the same PTFE type, and Fomblin(R)
30Z 25 oil, having a viscosity of 250 cSt.
The formulation comprises 68.5% by weight of
Fomblin(R) 2 25 and 31.5% by weight of PTFE.
z-r
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CHEMICAL-PHYSICAL CHARACTERIZATION
OF THE FORMULATED GREASE
With Fomblin With Fomblin
Consistency Z 1600z 25
Macropenetration
mm/10 (ASTM D 217) 283 286
Macropenetration after 10,000
double shots, mm/10 (ASTM
D 217) 270 298
Oil separation @204C/30 h
(FTMS 791-321) 1.6%10
Oil evaroration @204C/22 h
(ASTM D 972) 0.06% 0.1~
Relatively to the perfluorinated greases of the
prior art, the very low degree of oil separation and
evaporation results evident.
TRIBO-RHEOLOGICAL CHARACTERISTICS OF THE FORMULATED GREASE
The rheological characterization of the grease was
carried out by means of measurements of its dynamic
viscosity by using the plate/cone system mounted on a
Rotovisco HAAKE 12 system, with M150 head (PKII/0.3).
The first type of measurements was carried out at
25C with increasing y (flow gradient) values (20; 40; 80
sec 1), with the viscosity of the grease being measured at
start-up, and after 15 minutes. At each measurement, the
used grease was replaced by fresh grease.
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For a grease according to the invention,
formulated with Fomblin(R) z 1600, an early transient period
was observed, the duration of which increases with
decreasing ~ (from 5 to 6 minutes within the selected
y range). During this period, the grease shows a rheopexi~
behaviour, followed by a stabilization of the signal at an
equilibrium value higher than the initial values. The
oscillations of the signal are, under these conditions,
lower than of Z greases having a lower viscosity. The
dynamic viscosity of the investigated grease at 80 sec 1 is
equal to about 1,100 p (poises) whilst that of a grease
formulated with Fomblin(R) Z 25 oil is of about 260 p under
the same
~r
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conditions.
The behaviour of the viscosity over time, with
constant values, shows hence that the grease according to
the invention does not offer, at start-up, a higher
resistance than under equilibrium conditions. This result,
important in connection wi-th the power that the molar will
have to apply at start-up, never occurred wi-th greases
containing lower-viscosity PFPE-based oils.
The limited range of signal oscillation is0 indicative of a better constancy of lubricating performance.
A second type of measurement was carried out under
constant conditions of 40 sec 1, at temperatures of 25C;
50C, 100C. The endurance of the lubricating grease, at
100C, is exceptionally good, whilst, under the same
conditions, a grease formulated with a lower-viscosity oil
(Fomblin(R) Z 25) shows a trend to unhomogeneousness.
On the grease formulated according to the
invention a tribological evaluation was carried out, by
determining the welding load according to ASTM D 2596
Standard.
The welding load results higher than 700 kg
(device measurement limit).
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