Note: Descriptions are shown in the official language in which they were submitted.
~2W548
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FIELD OF THE INVENTION
This invention relates to a synthetic lubricating
fluid comprising a mixture or blend of a synthetic
naphthenic ester and a poly- ~-olefin. This synthetic
lubricating fluid is particularly suitable for use in power
transmissions.
BACKGROUND OF THE INVENTION
Conventional lubricating fluids for power
transmissions include a fluid comprised mainly of a mineral
oil, a fluid comprised mainly of a naphthenic synthetic oil
or poly-~ -olefinic synthetic oil, and a fluid comprised of
a mixture of two or more mineral oils or synthetic oils.
Of these conventional lubricating fluids, the fluid
comprised mainly of a naphthenic synthetic oil has been
extensively studied because of its higher performance in
power transmissions.
It is known that a hydrogenation product of ~ -
alkylstyrene dimer exhibits the highest power transmission
performance of all presently commercially available
synthetic lubricating fluids. A representative example of
such a synthetic lubricating fluid is "Santotrack*"
manufactured by the Mansanto Chemical Company and disclosed
in, e.g., Japanese Patent Laid-Open No. 7664/1972. It has
been suggested that the traction coefficient of a traction
fluid can be increased if the fluid comprises a mixture of
at least two components rather than a single component.
Therefore, attempts have been made to prepare a synthetic
fluid by mixing the above-mentioned hydrogenation product
of ~-alkylstyrene dimer or naphthenes, such as dimer,
trimer or codimer which is produced from styrene,
*Trade mark ~T~
12~9~4~
-- 2 --
alkylstyrene or its derivatives, with paraffinic hydrocarbons.
Such a lubricating fluid, is disclosed in Japanese Patent
Publication No. 35763/1972.
However, the commercially available hydrogenation
product of ~-alkylstyrene dimer has the disadvantage in
that the raw materials used for its production are rather
expensive. Furthermore, the present trend in the industry
is to develop smaller and more efficient power
transmissions. In order to fully utilize the performance
of newly developed power transmissions it is necessary to
further improve the traction coefficient of the synthetic
lubricating fluid used in these transmissions.
The term "traction coefficient" as used herein is
defined as the ratio of the tractional force which is
caused by slipping at the contact points between two
rotating members which are in contact with each other in a
power transmission of the rolling friction type to the
normal load.
In recent years there has been progress in the
development of continuously variable transmissions in the
automobile industry. In such a transmission the higher the
traction coefficient of the lubricating fluid the larger
the force transmitted. This allows a reduction in size of
the entire device with a corresponding reduction in the
emission of polluting exhaust gas. Therefore, there is a
strong demand for a lubricating fluid exhibiting the
highest possible traction coefficient.
In general, it was believed that mixing at least
two base oils could provide a fluid having a desired
traction coefficient because of the additive effect
discussed hereinafore. However, this additive effect does
not exist with all base oils. For example, it is known
that no additive improvement in performance is obtained
when a naphthenic synthetic oil is blended with about 5 to
50% by weight of a viscosity index improver such as
polymethacrylate.
B
1289548
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In other words, the performance of power trans-
missions of automobiles or tractors can be remarkably
enhanced if a synthetic lubricating fluid having a high
traction coefficient is combined with another component
which does not exhibit a high traction coefficient when
used alone but exhibits a noticeable synergistic effect
when used together with the fluid. However, such a
synthetic lubricating fluid has not yet been developed.
The present inventors have made extensive and
intensive studies with a view to developing a fluid having
a high traction coefficient for use as a lubricating fluid
in power transmissions. The present inventors have found
that the addition of a specific amount of a branched poly-
~-olefin to a synthetic naphthenic ester can meet the
above-mentioned requirements. The present invention is
based on this finding.
SUMMARY OF THE INVENTION
. _
The instant invention is directed to a synthetic
lubricating fluid for power transmissions comprising (i)
an ester comprised of the reaction product of cyclohexanol
and cyclohexanecarboxylic acid, and (ii) a branched
poly-~ -olefin.
DETAILED DESCRIPTION OF THE INVENTION
Specifically, in accordance with the present
invention there is provided a synthetic lubricating fluid
comprising an ester or its derivative of cyclohexanol with
cyclohexanecarboxylic acid or its derivative (hereinafter
often referred to as "component A") and a branched
pOly-a -olefin (hereinafter often referred to as "component
B"). The fluid of the present invention is characterized
in that t~e content of component B is 1 to 70~ by weight
based on the total weight of the fluid.
The ester of cyclohexanol and cyclohexanecarboxylic
1289548
-- 4
acid which constitutes component A is represented by the
following general formula:
R~ C- O-~
wherein Rl, R2, and R3 independently selected from a
hydrogen atom or an alkyl group having 1 to 8 carbon atoms.
The above-mentioned ester can be produced by the
following method. Cyclohexanecarboxylic acid or its
derivative and cyclohexanol or its derivative are used as
the reactants. The reaction is allowed to proceed using an
excess amount of the alcohol in the presence of a catalyst
such as phosphoric acid, while an inert gas such as
nitrogen is blown into the reaction system.
The cyclohexanecarboxylic acid or its derivative
reactant is a carboxylic acid having the above-mentioned
general formula in which Rl is a hydrogen atom or an
alkyl group having 1 to 8 carbon atoms. Examples of such a
carboxylic acid include cyclohexanecarboxylic acid,
methylcyclohexanecarboxylic acid, ethylcyclohexane-
carboxylic acid, propylcyclohexanecarboxylic acid, and
isopropylcyclohexanecarboxylic acid. A particularly pre-
ferred carboxylic acid is cyclohexanecarboxylic acid. On
the other hand, the cyclohexanol or its derivative reactant
is a straight-chain or branched alkyl substituted
cyclohexanol having the above-mentioned general formula in
which R2 and R3 are each a hydrogen atom or an alkyl
group having 1 to 8 carbon atoms.
Examples of the cyclohexanol include cyclohexanol,
methylcyclohexanol, ethylcyclohexanol, propylcyclohexanol,
isopropylcyclohexanol, and dimethylcyclohexanol, among
which cyclohexanol is particularly preferred.
Phosphoric acid, p-toluenesulfonic acid, sulfuric
acid or the like is used as the catalyst. The most
preferred catalyst is phosphoric acid because it enhances
1289~8
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the reaction rate and increases theyield of the ester.
In the esterification reaction the alcohol is used
in an amount of 1.5 to 3 times, by mole, that of the acid.
The amount of the catalyst is preferably 0.2 to 2% by
weight based on the total amount of the alcohol and acid.
The reaction temperature is 150 to 250C, preferably 170 to
230C, and the reaction time is 10 to 40 hr, preferably 15
to 25 hr. Water formed and evaporated during the
esterification reaction is trapped so as not to return to
the reaction vessel. The reaction is terminated when an
equimolar amount, with respect to the acid, of the water
has evaporated.
After completion of the reaction the reaction
mixture is washed with an aqueous sodium hydroxide solution
or the like until the reaction mixture is alkaline, and
then washed with water until the reaction mixture is
neutral. The reaction mixture is finally distilled under
reduced pressure to remove the excess alcohol. When the
pressure is 2 to 3 mmHg the alcohol distils at about 60 to
100C.
The ester thus produced has a kinematic viscosity
of 5 to 10 cst at 40C and has a traction coefficient about
5 to 7~ higher than commercially available traction base
oils having a viscosity in the same range. Therefore, the
above-mentioned ester compares favorably with conventional
commercially available fluids when it is used in a device
which requires a high traction coefficient in a relatively
low viscosity region such as textile producing machines and
food processing machines.
However, the use of the ester constituting
component A alone is unsatisfactory in applications where a
higher traction coefficient is required in a higher
viscosity region,e.g., automatic transmissions of
automobiles and large machinery for civil engineering uses
such as tractors. In such cases it is required that a
branched poly- a-olefin, component B, be used in
conjunction with component A.
12895~8
-- 6
The poly-~ -olefin component B has a quaternary
carbon atom or a tertiary carbon atom in its main chain,
and is a polymer of an c~-olefin having 3 to 5 carbon atoms
or the hydrogenation product thereof. Examples of the
poly-~r-olefin include polypropylene, polybutene, poly-
isobutylene and polypentene and the hydrogenation products
thereof. Particularly preferred are polybutene and
polyisobutylene and the hydrogenation products thereof.
The polyisobutylene is represented by the following
structural formula:
IC H 3 ,C H 3 ,C H 3
C H3 - C - C H2 tC - C H2 ~n C = C H2
C H3 C H3
The hydrogenation product of the polyisobutylene is
represented by the following structural formula:
IC H3 C, H3 ~C H3
C H3-- C-- C H2 ~C -- C H2 ~ C H--C H
C H3 C H3
In the above-mentioned formulae, the degree of polymerization,
n, is 6 to 200.
Although the polyisobutylene is generally
commercially available, it may be produced by conventional
polymerization methods. The hydrogenation product thereof
20 i8 produced by re~cting polyisobutylene in the presence of
hydrogen. The molecular weight of the polyisobutylene and
hydrogenation product thereof is preferably in the range of
500 to 10,000, more preferably in the range of 900 to
5,000. The molecular weight can be adjusted by suitable
methods such as decomposition of polyisobutylene having a
high molecular weight and mixing of low-molecular
polyisobutylenes.
Component B of the present invention, e.g.,
polyisobutylene, exhibits a traction coefficient of 0.075
to 0.085. Therefore, component B can be used alone in
conventional power transmissions without posing any
problem.
1289S~8
-- 7 --
However, it is projected that a traction coefficient
of 0.1 or more will be required in future traction drive
devices. The use of component B alone in such future
devices cannot satisfy this projected target. In such a
case it is necessary to use a synthetic lubricating fluid
prepared by blending the above-mentioned ester, i.e.,
component A, with the poly-~ -olefin, i.e., component B.
It has been found that a blend of the ester of the
instant invention with 1 to 70% by weight, particularly
preferably 10 to 50% by weight, of the poly- ~-olefin of
the instant invention exhibits an unexpectedly high
traction coefficient.
The synthetic lubricating fluid of the present
invention can be used in power transmissions of internal
combustion engines of automobiles and tractors, and general
industrial machinery such as textile machines and food
processing machines. When the transmission is a traction
drive device the synthetic lubricating fluid of the present
invention exhibits a remarkably enhanced performance.
Various additives may be added to the synthetic
lubricating fluid of the present invention. Specifically,
the addition of small amounts of a viscosity index
improver, antioxidant and anticorrosive contributes to
stabilization of the synthetic lubricating fluid of the
present invention over long periods of time in power
transmissions.
However, when a polymer which does not contain a
quaternary carbon atom of the gem-dimethyl type in its main
chain, such as polymethacrylate, is used as the viscosity
index improver the traction coefficient is lowered.
Therefore, if such a polymer is present, the amount of such
polymer should be less than about 10% by weight.
The reason why the synthetic lubricating fluid of
the present invention exhibits a traction coefficient much
higher than that of the conventional fluids is not yet
fully understood. However, basically the reason is
1289548
-- 8 --
believed to reside in the unique molecular structure of
both of the components, i.e., components A and B.
Specifically, the ester component A has two cyclohexyl
rings in its molecule linked through a carbonyl group. This
brings about an interdipolar force between the molecules.
It is believed that the interdipolar force serves to bring
the fluid into a stable glassy state under high-load
conditions, thereby increasing the shearing force. On the
other hand, the poly-~ -olefin, component B, is a nonpolar
hydrocarbon. Therefore, in a fluid, the force between the
molecules is mainly a van der Waals force. However,
component B has an appropriate molecular weight and
viscosity and possesses a number of branchings including a
gem-dimethyl quaternary carbon, which suppresses the
intramolecular rotation necessary for fluidization, thereby
resulting in high traction coefficient. Further, it is
believed that when the components A and B are blended
together very high traction coefficient can be attained
under high-load conditions. This is due to the fact that,
component A, having a cyclohexyl ring is firmly, engaged,
like gears, with component B having a gem-dimethyl
quaternary carbon.
The following Examples are provided for
illustrative purposes only and are not to be construed as
limiting the invention herein described.
EXAMPLES 1-2
Component A (an ester compound of cyclohexanol
with cyclohexanecarboxylic acid) of the present invention
was synthesized by the following method.
513 g t4 mol) of cyclohexanecarboxylic acid, 800 g
(8 mol) of cyclohexanol, and 10 g of phosphoric acid
(catalyst) were placed in a round-bottom flask. The
contents of the flask were heated at 200C for 20 hr while
nitrogen was blown into the flask. Water formed during the
1289548
g
esterification was trapped so as not to return to the
flask. The reaction was terminated when the amount of the
water reached 4 mol (72 cc).
After termination of the reaction the reaction
product was washed with an aqueous sodium hydroxide
solution until it was alkaline, thereby removing the
unreacted cyclohexanecarboxylic acid and phosphoric acid.
Subsequently, the reaction product was washed with water
until it was neutral to remove sodium hydroxide, followed
by distillation under reduced pressure to remove the excess
cyclohexanol, thereby obtaining component A, i.e., an ester
compound of cyclohexanol with cyclohexanecarboxylic acid.
The yield of the final product, i.e., component A, was
about 80%.
Thereafter, component A thus synthesized
(hereinafter referred to as "Al") and a second component
A (hereinafter referred to as "A2" which has been
synthesized in the same manner as mentioned above and has
the above general formula in which Rl is a methyl group
and R2 and R3 are each a hydrogen atom) were each
blended with polyisobutylene (component B), followed by
measurement of the traction coefficient. The measuring
equipment used was a Soda-type four roller traction testing
machine. The test was conducted under the following
conditions: a fluid temperature of 30C; a roller
temperature of 30C; a mean Hertzian pressure of 1.2 GPa; a
rolling velocity of 3.6 m/s; and a slipping ratio of 3.0%.
Under the above-mentioned conditions all oils exhibit the
maximum traction coefficient at a slipping ratio of about
3%. Therefore, the traction coefficient determined can be
regarded as the maximum traction coefficient. The
molecular weight and loadings of the polyisobutylene were
varied, followed by measurement of the traction coefficient.
The results are shown in Table 1.
~289548
-- 10 --
As can be seen from the results thus obtained a
high traction coefficient is attained when component A is
blended with 10 to 50% by weight of component B having a
molecular weight of 900 to 5000.
COMPARATIVE EXAMPLES 1-7
Traction coefficients were measured in the same
manner as described in the above examples with respect to
the following comparative samples: component A by itself
(Al or A2); component B by itself; a hydrogenation
product of ~ -methylstyrene linear dimer ("Santotrack"
manufactured by the Monsanto Chemical Company); a blend of
component Al with polymethacrylate (PMA); and a blend of
component Al with an ethylene-propylene copolymer (OCP).
The results are shown in Table 1. As can be seen
from Table 1 the traction coefficients of all the
comparative examples were 95% or less of the traction
coefficients of the synthetic lubricating fluid of the
present invention.
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128954~
- 13 -
The synthetic lubricating fluid of the present
invention which comprises a blend of an ester compound of
cyclohexanol with cyclohexanecarboxylic acid and l to 70%
by weight of a branched poly-~ -olefin having a molecular
weight of 500 to lO,OOO, exhibits an extremely high
traction coefficient. Therefore, the use of the synthetic
lubricating oil of the present invention in a power
transmission, particularly a traction drive device, results
in a remarkable increase in shearing force under a high
load. This allows a reduction in size of power transmissions
as a well as efficient operation of the transmissions.
