Thermal and oxidative stability of fluorinated alkyl aryl ethers

Article abstract of DOI:10.1016/S0022-1139(99)00196-7

A series of compounds composed of hydrocarbon aryl components and fluorinated alkyl components joined by ether linkages were developed for evaluation as high performance lubricants. Thermal and oxidative stability of these compounds were evaluated. The basic molecular structure was shown to be highly durable against high temperature, even in air, which is an indispensable property if it is to be suitable as base stock for high performance lubricants. However, thermal and oxidative stability decreased when hydrogen atoms were present on carbons adjacent to benzene rings, especially in the case of the carbon at the center of the compound, which is adjacent to two benzene rings.

Full text of DOI:10.1016/S0022-1139(99)00196-7

Journal of Fluorine Chemistry 101 (2000) 91±96
Thermal and oxidative stability of ¯uorinated alkyl aryl ethers
Hiroyuki Fukui*, Ken-ichi Sanechika, Masashi Watanabe, Masanori Ikeda
Asahi Chemical Industry Co., Ltd., Corporate R & D Administration, Hibiya-mitsui bld., 1-1-2 Yuraku-cho, Chiyoda-ku, Tokyo, Japan
Received 7 May 1999; received in revised form 21 July 1999; accepted 17 August 1999
Abstract
A series of compounds composed of hydrocarbon aryl components and ¯uorinated alkyl components joined by ether linkages were
developed for evaluation as high performance lubricants. Thermal and oxidative stability of these compounds were evaluated. The basic
molecular structure was shown to be highly durable against high temperature, even in air, which is an indispensable property if it is to be
suitable as base stock for high performance lubricants. However, thermal and oxidative stability decreased when hydrogen atoms were
present on carbons adjacent to benzene rings, especially in the case of the carbon at the center of the compound, which is adjacent to two
benzene rings. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: Thermal stability; Oxidative stability; Fluorinated alkyl aryl ethers; Synthetic lubricants
1. Introduction
stability and good viscosity properties [1], but their rela-
tively high production costs limit their use [14,15].
Fluorinated alkyl aryl ethers are considered to be
suitable for base stock of high performance lubricants
because they are expected to have (a) high stability, (b)
good lubricity, and (c) good miscibility with conventional
additives for hydrocarbon base stock (Fig. 1). In this
study, the thermal and oxidative stability of a series of
novel ¯uorinated alkyl aryl ethers developed as candidates
for base stock of high performance lubricants were inves-
tigated.
As performance and energy ef®ciency demands on
mechanical apparatus are increasingly heightened, require-
ments for lubricant performance are becoming correspond-
ingly demanding. Advanced aircraft and aerospace systems
place extreme thermal and oxidative stress on lubricants, and
maximum ¯uid temperatures in excess of 2608C (5008F)
have been estimated for some future applications [1±2].
Improvements in the energy ef®ciency of automobile
engines are also expected to require lubricants which
are highly durable at high temperature [3±4]. Air conditioner
and refrigeration systems also require oil with exceptional
thermal stability [5] in addition to solubility with refri-
gerants.
2. Experimental
Although some conventional lubricants and recently
developed candidate ¯uids have excellent thermal stability,
each of them have certain drawbacks which inhibit their
widespread adoption. Fluorinated polyethers have excellent
thermal and oxidative stability up to 3708C as well as good
lubricating properties [6±9], but high production costs limit
their use. Polyphenylether have excellent thermal and oxi-
dative stability also [10], but they exhibit poor lubricating
properties [3,11] and poor wetting characteristics [12].
Esters and silicones are not oxidatively stable at 2608C
[13]. Silahydrocarbons are reported to have superior thermal
2.1. Synthesis of fluorinated alkyl aryl ethers
The ¯uorinated alkyl aryl ethers investigated in this study
were synthesized as follows.
2.1.1. Synthesis of 2,2-bis[4-(1,1,2,2-tetrafluoroethoxy)-
phenyl]propane (hereinafter reffered to as ``BisA-TFE'')
A solution consisting of 68.7 g of 2,2-bis(4-hydroxyphe-
nyl)propane (hereinafter reffered to as ``bisphenol A'') and
6.2 g of potassium hydroxide dissolved in 120 ml of
dimethyl sulfoxide was charged into a 500 ml microcylinder.
The cylinder was degassed, charged with N2 gas to restore
atmospheric pressure, and then heated to 608C by means of
an oil bath. Tetra¯uoroethylene was then introduced to
initiate a reaction. The reaction was carried out for about
*Corresponding author. Tel.: ‡81-3-3507-2234; fax: ‡81-3-3507-2426.
E-mail address: a8411115@ut.asahi-kasei.co.jp (H. Fukui)
0022-1139/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 1 9 6 - 7

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H. Fukui et al. / Journal of Fluorine Chemistry 101 (2000) 91±96
colorless, transparent oil was obtained. The main fraction
(118 g) was isolated by simple distillation, at a boiling point
of about 1508C under about 0.1±0.3 mm Hg, indicating that
the yield was 92%.
The isolated fraction was analyzed by infrared absorption
‡
spectrometry and mass spectrometry [m/e 428 (M ), 413
‡
(M CH3)] to con®rm that the fraction was BisA-TFE,
having the structural formula shown in Fig. 2.
2.1.2. Synthesis of 1,1,3,3-tetramethyl-4-(1,1,2,2-
tetrafluoroethoxy)phenylbutane (hereinafter reffered
to as ``PTOP-TFE'')
PTOP-TFE was prepared in substantially the same man-
ner as BisA-TFE except that p-tert-octylphenol (obtained
from Tokyo Kasei Kogyo) was used in place of bisphenol A.
The yield was 94%. Infrared absorption spectrometry and
mass spectrometry [m/e 306 (M )] con®rmed that the
obtained product was PTOP-TFE, having the structural
formula shown in Fig. 2.
Fig. 1. Rationale for selection of fluorinated alkyl aryl ether as candidates
for base stock of high performance lubricants (R: alkyl group; Rf:
fluorinated alkyl group).
‡
5 h with tetra¯uoroethylene fed in to maintain reactor
pressure between 3 and 4 kg/cm2.
2.1.3. Synthesis of 2,2-bis[4-(1,1,2,2-tetrafluoroethoxy)-
phenyl]-4-methylpentane (hereinafter reffered to as
``MIBK-Bis-TFE'')
After completion of the reaction, reaction product was
obtained by distilling off dimethyl sulfoxide at 808C under
about 5 mm Hg, and 500 ml of 1,1,2-trichloro-1,2,2-tri¯uor-
oethane (hereinafter reffered to as ``CFC-113'') was added
to the reaction product obtained. This CFC-113 phase was
washed three times with 500 ml of distilled water, and CFC-
113 was removed off under reduced pressure. Thus, 130 g of
MIBK-Bis-TFE was prepared in substantially the same
manner as BisA-TFE except that 2,2-bis(4-hydroxyphenyl)-
4-methylpentane (obtained from Honshu Chemical) was
used in place of bisphenol A. The yield was 95%. Infrared
absorption spectrometry and mass spectrometry [m/e 470
Fig. 2. Structure and synthesis of fluorinated alkyl aryl ethers.

H. Fukui et al. / Journal of Fluorine Chemistry 101 (2000) 91±96
93
‡
(M )] con®rmed that the obtained product was MIBK-Bis-
TFE, having the structural formula shown in Fig. 2.
2.1.4. Synthesis of BisP-OT-TFE, BisOTBP-A-TFE,
BisOSBP-A-TFE, BisTOP-F-TFE, BisP-IOTD-TFE and
TMBisA-TFE
These compounds were prepared in substantially the same
manner as BisA-TFE, except that 2,2-bis(4-hydroxyphenyl)-
octane, 2,2-bis(3-tert-butyl-4-hydroxyphenyl)propane, 2,2-
bis(3-sec-butyl-4-hydroxyphenyl)propane, bis(tert-octylhy-
droxyphenyl)methane, 1,1-bis(4-hydroxyphenyl)-2-ethyl-
hexane, and 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane
were used in place of bisphenol A. Each of these materials
were obtained from Honshu Chemical, except bis(tert-
octylhydroxyphenyl)methane, which was obtained from
Asahi Organic Chemicals. Yields were 90%, 91%, 94%,
95%, 88%, and 46%, respectively. Infrared absorption
spectrometry and mass spectrometry con®rmed that the
products were compounds having the structural formulas
shown in Table 2.
Fig. 3. High-pressure DSC chart of BisA-TFE.
2.2.3. Measurement of total acid number (TAN) and
degree of decomposition
2.2. Evaluation of thermal and oxidative stability
Specimens were heated to 1758C for 19 h in an atmo-
sphere of N2 with 1 wt% O2, standard conditions for a
preliminary test of stability for refrigeration system lubri-
cants. TAN was then measured in substantially the same
manner as ISO 6618, except that 0.01 mol/l solution of
potassium hydroxide was used in place of 0.1 mol/l solution.
Degree of decomposition was determined through gas
chromatography, expressed by area of peaks of thermal
decomposition products as percentage of total gas chroma-
tograph area.
Thermal and oxidative stability of the ¯uorinated alkyl
aryl ethers were measured, and their results were compared
with those obtained for commercially available lubricants.
2.2.1. Commercially available lubricants
Three types of per¯uoropolyether oils, as below, were
studied.
Fluid K: C3F7O[CF(CF3)CF2O]xC2F5.
Fluid F: CF3O[CF(CF3)CF2O]m(CF2O)nCF3.
Fluid D: C3F7O(CF2CF2CF2O)xC2F5.
3. Results and discussion
Fluid K was obtained from DuPont (Krytox 143AB),
Fluid F was obtained from Monte¯uos (Fomblin Y25A) and
Fluid D was obtained from Daikin Industries (Demnun
S-65).
Two typical polyphenylethers were obtained from Mat-
sumura Oil Research: m-bis(m-phenoxyphenoxy)benzene
(S-3105), and monoalkyl-m-phenoxy-o-biphenyl (S-
3101). Dimethylsilicone (KF96-100cSt), phenylmethylsili-
cone (KF-54) and ¯uorinated silicone (X-22-822) were
obtained from Shin-Etsu Chemical. Branched alkylbenzene
(Aromix 20T) was obtained from Nippon Petroleum Deter-
gent.
3.1. Thermal and oxidative stability of the basic molecular
structure
Fig. 4 shows the basic molecular structure of the ¯uori-
nated alkyl aryl ethers studied. It consists of a hydrocarbon
aryl moiety, a ¯uorinated alkyl moiety and an ether linkage
group. In order to evaluate thermal and oxidative stability of
this basic molecular structure, BisA-TFE, PTOP-TFE, and
MIBK-Bis-TFE (shown in Fig. 2) were prepared by nucleo-
philic additions of tetra¯uoroethylene (TFE) to alkylphenol
or bisphenols.
2.2.2. Measurement of decomposition temperature
Decomposition temperatures were investigated using a
high-pressure differential scanning calorimeter manufac-
tured by Rigaku. Sample temperatures were raised in the
order of 58C/min up to 4008C. Decomposition temperatures
were determined by observing the predominant slope of the
®rst peak to appear, and extending a line from this slope
back to the baseline, as shown in Fig. 3.
Fig. 4. Basic molecular structure of fluorinated alkyl aryl ether (R: alkyl
group; Rf: fluorinated alkyl group).

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H. Fukui et al. / Journal of Fluorine Chemistry 101 (2000) 91±96
Table 1
Decomposition temperature of fluorinated alkyl aryl ethers and conventional high performance lubricants
Sample
Viscositya (cSt at 408C)
Decomposition temperature (8C)b
In air
In N2
Fluorinated alkyl aryl ether
BisA-TFE
PTOP-TFE
26
6
347
300
290
>400
>400
>400
MIBK-Bis-TFE
109
Perfluoropolyether
Fluid Dc
65
86
94
388
384
375
>400
367
Fluid Kd
Fluid Fe
>400
Polyphenylether
m-bis(m-phenoxyphenoxy)benzene (S-3105)
Monoalkyl-m-phenoxyphenoxy-o-biphenyl (S-3101)
Silicone oil
266
243
352
210
>400
>400
Dimethylsilicone (KF96-100cSt)
Phenylmethylsilicone (KF-54)
Fluorinated silicone (X-22-822)
100
400
100
252
345
235
>400
>400
247
a Measured with E-type rotational viscometer (Tokyo Keiki).
b Measured with high-pressure DSC (Rigaku).
c Demnum S-65, manufactured by Daikin.
d Krytox 143AB, manufactured by E.I. DuPont De Nemours.
e Fomblin Y25A, manufactured by Nippon Montedison K.K.
Table 1 shows decomposition temperatures that resulted
from high-pressure DSC analysis. In addition to these three
¯uorinated alkyl aryl ethers, a representative range of per-
¯uoropolyethers, polyphenylethers, and silicone oils, which
are conventionally used as lubricants in extreme heat con-
ditions, were also evaluated.
In nitrogen, ¯uorinated silicone was the only sample
which showed an exothermic peak below 4008C. In contrast,
all samples showed exothermic peaks under 4008C in air,
indicating oxidative decomposition. Monoalkyl-m-phenox-
yphenoxy-o-biphenyl, ¯uorinated silicone, and dimethyl
silicone were shown to be the least resistant to oxidative
decomposition, with decomposition temperatures of 2108C,
2358C, and 2528C, respectively.
The ¯uorinated alkyl aryl ethers proved to be quite stable
against high temperatures, with none of the samples show-
ing endothermic or exothermic peaks below 4008C in
nitrogen, while exothermic peaks in air were observed at
3008C, 3478C, and 2908C for PTOP-TFE, BisA-TFE, and
MIBK-Bis-TFE, respectively, all higher oxidative decom-
position temperatures than observed with monoalkyl-m-
phenoxyphenoxy-o-biphenyl, ¯uorinated silicone and
dimethyl silicone. These results indicated that the basic
¯uorinated alkyl aryl ether molecular structure studied was
highly durable against high temperature, even in air.
N2 containing 1 wt% O2. Table 2 shows changes in TAN and
degree of decomposition.
Both change in TAN and degree of decomposition for
BisA-TFE and MIBK-Bis-TFE were very slight, indicating
good stability under these test conditions. BisOTBP-A-TFE,
BisOSBP-A-TFE and TMBisA-TFE are compounds with
tert-butyl groups, sec-butyl groups and methyl groups,
respectively, substituted on the benzene rings of BisA-
TFE. BisOTBP-A-TFE, with tert-butyl group substitutions,
displayed TAN and decomposition results similar to BisA-
TFE. BisOSBP-A-TFE, with sec-butyl group substitutions,
showed slightly more decomposition, but similar TAN
results. In contrast, TMBisA-TFE, with methyl group sub-
stitutions, showed a TAN increase to 1.26 mg KOH/g and
3.62% decomposition.
These results suggest that thermal and oxidative stability
is related to the presence of hydrogen atoms on carbon
atoms adjacent to the benzene rings. Such carbons appear to
be susceptible to oxidation under high temperature. Gas
chromatography±mass spectroscopy analysis of post-test
sample from BisOSBP-A-TFE revealed a peak at 526 m/e.
Itisbelievedthatthispeakcorrespondstoanoxidationproduct
with the structure shown in Fig. 5, with the oxidation occurr-
ing at one of the carbons adjacent to a benzene ring.
3.2. Effect of substituents on thermal and oxidative
stability
In order to evaluate the effect of alkyl-group substituents
on the thermal and oxidative stability of BisA-TFE, a series
of compounds were prepared, and tested at 1758C for 19 h in
Fig. 5. Proposed structure for decomposition product of BisOSBP-A-TFE.

H. Fukui et al. / Journal of Fluorine Chemistry 101 (2000) 91±96
95
Table 2
Stability of substituted fluorinated alkyl aryl ether at test condition: 1758C, 19 h in N2 containing 1 wt% O2
Compounds
Viscosityb (cSt at 408C)
TAN(mg KOH/g)
Degree of decompositiona (%
GC area)
Before test
After test
BisA-TFE
26
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.01
<0.01
0.02
<0.01
<0.01
0.04
<0.01
0.03
2.06
2.14
3.62
±
MIBK-Bis-TFE
BisP-OT-TFE
109
112
BisOTBP-A-TFE
BisOSBP-A-TFE
BisTOP-F-TFE
BisP-IOTD-TFE
TMBisA-TFE
(Solid)
91
<0.01
0.01
1789
250
0.25
0.35
1.26
(Solid)
14
c
Branched alkyl benzene
1.55
a Gas chromatography (area % of peaks for thermal decomposition products, based on the total area of chromatogram).
b Measured with E-type rotational viscometer (Tokyo Seiki).
c Aromix 20T (Nippon Petroleum Detergent)
.
BisP-OT-TFE, a compound with an n-hexyl group sub-
These conclusions will serve as a useful guide to maintain
stability while introducing various substituent groups to
¯uorinated alkyl aryl ethers in order to improve other
properties. In subsequent studies, the authors will evaluate
lubricity and other properties of ¯uorinated alkyl aryl ethers
for use as lubricants.
stituted on the central carbon of BisA-TFE, turned out to be
highly stable under the test conditions. Both BisTOP-F-
TFE, with two hydrogen atoms on the central carbon and
tert-octyl groups substituted on the benzene rings, and BisP-
IOTD-TFE, with one hydrogen atom and one 1-ethyl pentyl
group on the central carbon, displayed TAN and decom-
position results which indicate susceptibility to oxidative
decomposition. It is believed that BisTOP-F-TFE and BisP-
IOTD-TFE were highly susceptible to oxidation due to the
presence of hydrogen atoms on the central carbon, which is
adjacent to benzene rings on two sides.
Acknowledgements
The authors are grateful to Dr. Nobuto Hoshi, Mr. Hiroshi
Murata and Mr. Atsushi Seo of Asahi Chemical for their
helpful discussion.
4. Conclusions
References
These results suggest the following conclusions: (1) the
basic molecular structure of ¯uorinated alkyl aryl ethers has
excellent oxidative and thermal stability, but (2) thermal and
oxidative stability decreases when hydrogen atoms are
present on carbon atoms adjacent to benzene rings.
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