Thermally stable perfluoropolyethers and processes therefor...

Details Thermally stable perfluoropolyethers and processes therefor... Thermally stable perfluoropolyethers and processes therefor...

2001-07-10

2004-06-22

Toomer, Cephia D. (Department: 1714)

Solid anti-friction devices, materials therefor, lubricant or se

Lubricants or separants for moving solid surfaces and...

Organic oxygen compound

C252S380000, C568S615000

Reexamination Certificate

active

06753301

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a perfluoropolyether having improved thermostability over the presently available perfluoropolyethers, to a process therefor, and to a process therewith.
BACKGROUND OF THE INVENTION
Hereinafter trademarks or trade names are shown in upper case characters.
Perfluoropolyethers (hereinafter PFPE) are fluids having important uses in oils and greases for use under extreme conditions. A property shared by the class is extreme temperature stability in the presence of oxygen and they find use in tribological or lubrication applications. Among their advantages as extreme lubricants is the absence of gums and tars among the thermal decomposition products. In contrast to the gum and tar thermal degradation products of hydrocarbons, the degradation products of PFPE fluids are volatile. In actual use, the upper temperature limit is determined by the stability of the oil or grease. Lewis acids, metal fluorides such as aluminum trifluoride or iron trifluoride, are formed as a result of heat at microscale loci of metal to metal friction; for instance as stationary bearings are started in motion. Thus the PFPE stability in the presence of the metal fluoride, although lower than the stability in the absence of the metal fluoride, establishes the upper performance temperature. The three commercial PFPEs, KRYTOX (from E. I. du Pont de Nemours and Company, Inc., Wilmington Del.), FOMBLIN and GALDEN (from Ausimont/Montedison, Milan, Italy) and DEMNUM (from Daikin Industries, Osaka, Japan) differ in chemical structure. A review of KRYTOX is found in
Synthetic Lubricants and High
-
Performance Fluids
, Rudnick and Shubkin, Eds., Marcel Dekker, New York, N.Y., 1999 (Chapter 8, pp. 215-237). A review of FOMBLIN and GALDEN is found in
Organofluorine Chemistry
, Banks et al., Eds., Plenum, New York, N.Y., 1994, Chapter 20, pp. 431-461, and for DEMNUM, in
Organofluorine Chemistry
(op. cit.), Chapter 21, pp. 463-467.
The anionic polymerization of hexafluoropropylene epoxide as described by Moore in U.S. Pat. No. 3,332,826 can be used to produce the KRYTOX fluids. The resulting poly(hexafluoropropylene epoxide) PFPE fluids are hereinafter described as poly(HFPO) fluids. The initial polymer has a terminal acid fluoride, which is hydrolyzed to the acid followed by fluorination. The structure of a poly(HFPO) fluid is shown by Formula 1:
CF
3
—(CF
2
)
2
—O—[CF(CF
3
)—CF
2
—O]
s
—R
f
  (Formula 1)
where s is 2-100 and R
f
is a mixture of CF
2
CF
3
and CF(CF
3
)
2
, with the ratio of ethyl to isopropyl terminal group ranging between 20:11 to 50:1.
DEMNUM fluids are produced by sequential oligomerization and fluorination of 2,2,3,3-tetrafluorooxetane (tetrafluorooxetane), yielding the structure of Formula 2.
F—[(CF
2
)
3
—O]
t
—R
f
2
  (Formula 2)
where R
f
2
is a mixture of CF
3
or C
2
F
5
and t is 2-200.
A common characteristic of the PFPE fluids is the presence of perfluoroalkyl terminal groups.
The mechanism of thermal degradation in the presence of a Lewis acid such as aluminum trifluoride has been studied. Kasai (Macromolecules, Vol. 25, 6791-6799, 1992) discloses an intramolecular disproportionation mechanism for the decomposition of PFPE containing —O—CF
2
—O— linkages in the presence of Lewis acids.
FOMBLIN and GALDEN fluids are produced by perfluoroolefin photooxidation. The initial product contains peroxide linkages and reactive terminal groups such as fluoroformate and acid fluoride. These linkages and end groups are removed by ultraviolet photolysis and terminal group fluorination, to yield the neutral PFPE compositions FOMBLIN Y and FOMBLIN Z represented by Formulae 3 and 4, respectively
CF
3
O(CF
2
CF(CF
3
)—O—)
m
(CF
2
—O—)
n
—R
f
3
  (Formula 3)
where R
f
3
is a mixture of —CF
3
, —C
2
F
5
, and —C
3
F
7
; (m+n) is 8-45; and m
is 20-1000; and
CF
3
O(CF
2
CF
2
—O—)
p
(CF
2
—O)
q
CF
3
  (Formula 4)
where (p+q) is 40-180 and p/q is 0.5-2. It is readily seen that Formulae 3 and 4 both contain the destabilizing —O—CF
2
—O— linkage since neither n nor q can be zero. With this —O—CF
2
—O— linkage in the chain, degradation within the chain can occur, resulting in chain fragmentation.
For PFPE molecules with repeating pendant —CF
3
groups, Kasai discloses the pendant group provides a stabilizing effect on the chain itself and for the alkoxy end groups adjacent to a —CF(CF
3
)—. Absent the —O—CF
2
—O— linkage, the PFPE is more thermally stable, but its eventual decomposition was postulated to occur at end away from the stabilizing —CF(CF
3
)— group, effectively unzipping the polymer chain one ether unit at a time.
Therefore, there is substantial interest and need in increasing the thermal stability of PFPE fluids.
SUMMARY OF THE INVENTION
According to a first embodiment of the invention, a perfluoropolyether or a composition comprising thereof is provided, in which the perfluoropolyether comprises perfluoroalkyl radical end groups in which the radical has at least 3 carbon atoms per radical and is substantially free of perfluoromethyl and perfluoroethyl, and a 1,2-bis(perfluoromethyl)ethylene diradical, —CF(CF
3
)CF(CF
3
)—, is absent in the molecule of the perfluoropolyether.
According to a second embodiment of the invention, a process for improving the thermal stability of a perfluoropolyether is provided, which comprises modifying a process for producing a perfluoropolyether such that substantially all end groups of the perfluoropolyether have at least 3 carbon atoms per end group or, preferably, are C
3
-C
6
branched and straight chain perfluoroalkyl end groups.
According to a third embodiment of the invention, a process is provided for producing a perfluoropolyether comprising perfluoroalkyl radical end groups in which the perfluoroalkyl radical has at least 3 carbon atoms per radical as disclosed in the first embodiment of the invention. The process can comprise (1) contacting a perfluoro acid halide, a C
2
to C
4
-substituted ethylene epoxide, a C
3+
fluoroketone, or combinations of two or more thereof with a metal halide to produce an alkoxide; (2) contacting the alkoxide with either hexafluoropropylene oxide or 2,2,3,3-tetrafluorooxetane to produce a second acid halide; (3) esterifying the second acid halide to an ester; (4) reducing the ester to its corresponding alcohol; (5) converting the corresponding alcohol with a base to a salt form; (6) contacting the salt form with a C
3
or higher olefin to produce a fluoropolyether; and (7) fluorinating the fluoropolyether.
According to a fourth embodiment of the invention, a thermally stable grease or lubricant is provided, which comprises a thickener with a perfluoropolyether of composition thereof disclosed in the first embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention is directed to a thermal stable perfluoropolyether (or PFPE) composition and processes for making and using the composition. The term “perfluoropolyether” and “PFPE fluid” (“PFPE” or “PFPE fluids”) are, unless otherwise indicated, exchangeable.
According to the first embodiment of the invention, there is provided a perfluoropolyether comprising branched or straight chain perfluoroalkyl radical end groups, each of which has at least 3 carbon atoms per radical, is substantially free of perfluoromethyl and perfluoroethyl end groups and does not contain any 1,2-bis(perfluoromethyl)ethylene diradicals [—CF(CF
3
)CF(CF
3
)—] in the chain. The term “substantially”, as used herein, refers to a perfluoropolyether or PFPE fluid of this invention having only trace C
1
-C
2
perfluoroalkyl endgroups such that the initial decomposition in a specific use is inconsequential and tolerable. An unavoidable trace of remaining perfluoropolyether or PFPE molecules with a perfluoro-methyl or -ethyl end group, while not desirable, may be tolerable as such molecules degrade to volatile products, leaving the more stable PFPE molecules. Thus thermal stability increases after some initial degradation.
The preferred perfluoropolyethers

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