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Description  |
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FIELD OF THE INVENTION
This invention concerns lubricating oil compositions which are especially
adapted for use in mechanical systems where gears are subjected to great
stresses and extremely high pressures such as those found in automotive
rear axles and tractor transmissions. In particular, the invention
concerns a gear lubricant comprising in combination a base oil, at least
one polymer of specified structure and molecular weight, and at least one
extreme pressure or load-carrying agent.
BACKGROUND OF THE INVENTION
The problems associated with the lubrication of automotive rear axle and
transmission gears are well known to those of skill in the art. Such gears
are usually hypoid in nature and for reasons inherent in their design
develop great pressures between the contacting metal surfaces of the gear.
These pressures literally squeeze out ordinary mineral oil lubricants from
between the gears and result in metal-to-metal contact between the parts
of the gear under extremely high pressures. Such metal-to-metal contact
soon leads to deformation and destruction of the gear surfaces.
To alleviate these and other problems it has become a common practice to
add to gear oils certain chemical substances which are generally called
additives. Among such additives are extreme pressure agents which are
widely believed to react with the metal surfaces of the gear under the
elevated temperatures and high pressures generally encountered during
operation of the gear to form thin films. These newly formed layers act as
separants and lubricants for the gear surfaces and thus prevent or reduce
destructive metal-to-metal contact.
Recently a demand has arisen for lubricants which will function
satisfactorily as gear lubricants under a wide range of temperature
conditions. Such compositions are characterized by relatively small
changes in their viscosities with changing temperature and they are
commonly graded according to SAE standards according to their viscosities
at low (e.g., 0.degree. F.) and high temperatures (e.g., 210.degree. F.)
as is explained in further detail below. Because of this grading, such
gear oils are often said to be "multigraded." In terms of widely accepted
concepts such multigraded gear lubricants can also be said to have high
viscosity indices. Multigraded gear lubricants thus have the desirable
property of being able to function immediately, though cold, upon being
put into service and continue to function satisfactorily as they become
heated during operation.
In addition to being multigraded, it is desirable that lubricant
compositions especially adapted for use as gear lubricants, exhibit shear
stability. This means that they will not degrade or lose their viscosity
as a result of the shearing forces encountered during their use.
Compositions exhibiting desirable shear stability will be found to
generally have a viscosity within 85 to 95% of their original viscosity
after many hours (say 1000) of service. It has been recognized that many
ordinary viscosity index improvers commonly added to crankcase lubricating
oils, such as high molecular weight polyisobutylenes and polyacrylates, do
not possess the desired shear stability for use in improving the viscosity
properties of gear lubricants.
It has now been found that, in accordance with this invention, multigrade
lubricants exhibiting exceptional shear stabilities can be made by
combining in certain proportions base oils, polymers of specified
structure and molecular weight and extreme pressure agents.
SUMMARY OF THE INVENTION
The lubricating oil compositions of this invention comprise (A) about 95 to
about 30, preferably about 90 to about 50 weight percent of a base oil,
(B) at least one extreme pressure agent in an amount sufficient to impart
extreme pressure properties thereto, and (C) at least one oil-soluble
interpolymer, said interpolymer being a hydrogenated interpolymer of at
least one monovinyl arene and at least one C.sub.4-6 conjugated diene or
at least one C.sub.2-6 alpha-olefin. The interpolymer has a number average
molecular weight of about 750 to about 10,000, and is present in the
composition in an amount so as to allow the composition to be multigraded
according to SAE standards in the multigrade range between SAE 75W and SAE
250.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The base oils of the present invention, that is, component (A), have
viscosities of about 40 to about 2000 SUS at 100.degree. F. Preferably
they have viscosities of about 75 to about 500 SUS at 100.degree. F., and
most preferably of about 100 to about 300 SUS at 100.degree. F. These base
oils may be selected from natural oils, synthetic oils, and combinations
of two or more of these types of oils; e.g., combinations of different
natural oils, combinations of different synthetic oils, or combinations of
one or more natural oils with one or more synthetic oils. Selection of
materials suitable as base oils is within the skill of the art using the
parameters herein set forth.
When the base oil is a mixture of natural and synthetic oils, the mixture
contains about 5 to about 95 weight percent of at least one synthetic oil.
Preferably such mixtures contain 40 to about 90% natural oil and 60 to
about 10% synthetic oil. Of course, both the natural oils and synthetic
oils can themselves be mixtures of various types of natural oils or
synthetic oils.
When the base oil consists of natural oils, they are often derived from
petroleum, although oils derived from other natural sources such as shale,
coal, animals and plants can also be used. Among the general types of
petroleum oils useful in the compositions of this invention are solvent
neutrals, bright stocks, cylinder stocks, residual oils, hydrocracked base
stocks, paraffin oils including pale oils, and solvent extracted
naphthenic oils. Such oils and blends of them are produced by a number of
conventional techniques which are widely known by those of skill in the
art. All that is required of oils useful in this invention is that they
have viscosities falling within the above-noted ranges and be generally
compatible with the other components of the gear lubricant and the
materials used in construction of the gear assemblies in which they
function.
As is noted above, the base oil can consist essentially of or comprise a
portion of one or more synthetic oils. Among the suitable synthetic oils
are homo- and interpolymers of C.sub.2-12 olefins, carboxylic acid esters
of both monoalcohols and polyols, polyethers, silicones, polyglycols,
silicates, alkylated aromatics, carbonates, thiocarbonates, orthoformates,
phosphates and phosphites, borates and halogenated hydrocarbons.
Representative of such oils are homo- and interpolymers of C.sub.2-12
monohydrocarbyl olefins, alkylated benzenes (e.g., dodecyl benzenes,
didodecyl benzenes, tetradecyl benzenes, dinonyl benzenes,
di-(2-ethylhexyl) benzenes, wax alkylated naphthalenes, etc.); polyphenyls
(e.g., biphenyls, terphenyls, etc.); and the like.
Alkylene oxide polymers and interpolymers and derivatives thereof where the
terminal hydroxyl groups have been modified by esterification,
etherification, etc., constitute another class of useful synthetic oils.
These are exemplified by the oils prepared through polymerization of
alkylene oxides such as ethylene oxide or propylene oxide, the alkyl and
aryl ethers of these polyoxyalkylene polymers (e.g.,
methylpolyisopropylene glycol ether having an average molecular weight of
1000, diphenyl ether of polyethylene glycol having a molecular weight of
500-1000, diethyl ether of polypropylene glycol having a molecular weight
of 1000-1500, etc.) or mono- and polycarboxylic esters thereof, for
example, the acetic acid esters, mixed C.sub.3 -C.sub.8 fatty acid esters,
or the C.sub.13 Oxo acid diester of tetraethylene glycol. Among such
glycols are the polyalkylene glycols sold by the Union Carbide Corp. of
New York under trade names and marks of UCON LB-135, UCON LB-385, and UCON
LB-1145.
Another suitable class of synthetic oils comprises the esters of
dicarboxylic acids (e.g., phthalic acid, succinic acid, maleic acid,
azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid,
linoleic acid dimer, etc.) with a variety of alcohols (e.g., butyl
alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene
glycol, etc.). Specific examples of these esters include dibutyl adipate,
di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate,
diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl
phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid
dimer, the complex ester formed by reacting one mole of sebacic acid with
two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid
and the like. Among the halogenated hydrocarbons useful as base oils are
chlorinated phenyl and biphenyls, low molecular weight
chlorotrifluoroethylene polymers, polytetrafluoroethylenes of appropriate
molecular weights and the like.
Esters useful as synthetic oils also include those made from C.sub.5 to
C.sub.12 monocarboxylic acids and polyols and polyol ethers such as
neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol,
etc.
Silicon-based oils such as the polyalkyl-, polyaryl-, polyalkoxy-, or
polyaryloxy-siloxane oils and silicate oils comprise another useful class
of synthetic lubricants (e.g., tetraethyl silicate, tetraisopropyl
silicate, tetra-(2-ethylhexyl)silicate,
tetra-(4-methyl-2-tetraethyl)silicate, tetra-(p-tert-butylphenyl)silicate,
hexyl-(4-methyl-2-pentoxy)-disiloxane, poly-(methyl)-siloxanes,
poly(methylphenyl)-siloxanes, etc.). Among these are the oils sold by the
Dow Corning Co. of Michigan, under the trade names and marks of DC-200,
DC-500 and DC-700. Other synthetic lubricating oils include liquid esters
of phosphorus-containing acids (e.g., tricresyl phosphate, trioctyl
phosphate, triphenyl phosphite, diethyl ester of decane phosphonic acid,
etc.).
Organic carbonates and thiocarbonates such as those formed from the well
known Oxo alcohols as well as orthoformates, mercaptals, thiol or
thioformates and the like can also be used as synthetic oils in this
invention, if in the form they are used (e.g., as blends) they exhibit
viscosities within the aforesaid limits.
The fact that the base oils of this invention may consist of natural,
synthetic, or natural-synthetic blended oils does not mean that these
three types of oils are necessarily equivalents of each other. Certain
types of base oils may be used in certain inventive compositions for the
specific properties they possess such as nonflammability or lack of
corrosivity towards specific metals (e.g., silver or cadmium) which are
used in certain specialized gear assemblies. In other compositions, other
types of base oils may be preferred for reasons of availability or low
cost. Thus, the skilled artisan will recognize that while the three types
of base oils discussed above are all useful in the compositions of the
present invention, they are not necessarily functional equivalents of each
other in every instance.
As is clear from the above discussion, in certain embodiments of this
invention, the synthetic oil is not solely hydrocarbyl in nature, but can
contain atoms of oxygen, sulfur, silicon, halogen, etc., as well as those
of carbon and hydrogen; of course, in other embodiments the synthetic oil
will be solely hydrocarbyl in nature. This is also true of the natural
oils used in this invention.
It is preferable that, if the lubricants of the present invention consist
essentially of only one synthetic base oil and only one polymer, i.e.,
component (C), the base oil should not be derived from the same olefin as
the polymer. Thus, if the base oil is a synthetic styrene-butadiene
polymer, it is preferable that the polymer (C) not be a butadiene-styrene
polymer.
In many embodiments it is preferable that the synthetic oil have a number
average molecular weight below about 800. Naturally, such oils must also
conform to the viscosity limitations set forth above.
Another component of the lubricating oils of this invention, that is,
component (C), is at least one oil-soluble interpolymer derived from one
or more monovinyl arenes and at least one C.sub.4-6 conjugated diene
and/or at least one C.sub.2-6 alpha-olefin. The characterization of these
polymers as oil-soluble does not necessarily mean they are soluble in all
base oils in all proportions; it means they are soluble in the base oils
with which they are formulated to a degree sufficient to allow the
lubricant composition to be multigraded between SAE 75W and SAE 250. These
polymers have number average molecular weights within the range of about
750 to about 5,000; preferably their molecular weights lie between about
900 and about 3,000. As used herein, the term "interpolymer" is intended
to be generic to copolymers, terpolymers, and the like.
The monovinyl arene monomers from which the interpolymers (C) are derived
can comprise vinyl-substituted, mono-, di-, or polynuclear aromatic
compounds such as styrene, alpha-methyl styrene, a vinyl pyridine, vinyl
naphthylene, vinyl phenanthrene, etc. Preferably, however, they are
monovinyl mononuclear aromatic compounds such as styrene, mono- or
polyalkylated styrenes and substituted styrenes including substituted
alkylated styrenes. The alkyl groups in the alkylated styrenes can be
attached to the alpha carbon of the styrene (as, for instance, in
alpha-methyl styrene) and/or to the aromatic ring (as, for instance, in
o-, m-, or p-methyl styrene, paratertiary butyl styrene, etc.) and
normally will contain no more than about thirty carbon atoms; preferably
such alkyl groups, if present, will be lower alkyl groups of less than ten
carbon atoms. Styrene and para-tertiary butyl styrene are preferred as
monomers for formation of the interpolymer (C).
Mixtures of various vinyl aromatic monomers can also be used to produce
these interpolymers. Substituted monovinyl arene compounds can also be
used, wherein the arene portion of the molecule is substituted with, for
instance, an ether group (especially hydrocarbyloxy and particularly
alkoxy groups of about up to ten carbon atoms), amino groups (particularly
disubstituted amino groups such as dialkyl or diaryl amino and the like,
e.g., dimethyl amino, diheptyl amino, dicyclohexyl amino, etc.), nitro
groups, cyano groups, thioether groups (especially C.sub.1-10 alkyl
thioethers), and halo groups such as fluorine, chlorine, bromine, etc.
Examples of the C.sub.4 -C.sub.6 conjugated dienes which are useful in
producing the interpolymers of the present invention are piperylene,
2,3-dimethyl butadiene, chloroprene, isoprene and butadiene, the last two
being particularly preferred. Mixtures of such conjugated dienes, e.g.,
butadiene and isoprene, can also be used.
The C.sub.2 -C.sub.6 alpha-olefins which are useful in producing the
interpolymers (C) can be normal or branch-chain alpha-olefins such as
ethylene, propylene, n-butene-1, isoprene, 2-methyl pentene-1,
isobutylene, 3-ethyl pentene-1, n-hexene, etc.
The ratio of monovinyl arene and conjugated diene or alpha-olefin monomers
used to produce the interpolymers of the present invention can vary
between wide limits. But as it is noted above, after hydrogenation, the
interpolymer is soluble in the base oil into which it is incorporated.
This fact limits the amount of monovinyl arene monomer which can be
incorporated into the interpolymer, since the solubility of the
hydrogenated interpolymer can often depend on its particular structure. In
the case where the monovinyl arene monomer is styrene it is usually found
that no more than about seventy mole percent of styrene can be
incorporated into the interpolymer while when the monovinyl arene compound
is a substituted styrene (e.g., p-tertiary butyl styrene) often amounts in
up to 99 mole percent of the monovinyl arene compound can be incorporated.
Thus, interpolymers with ratios of the number of units originating from
the monovinyl arene compound to the number of units originating from a
conjugated diene or the alpha-olefin can vary between from about 1 to
about 99 and about 90 to 10, preferably between about 5 to about 95 and
about 85 to about 15, and more preferably between about 30 to about 70 and
about 75 to about 25. If desired, the interpolymers (C) can also contain a
relatively small amount of units derived from other monomers. Examples of
such other monomers which can be mentioned are vinyl pyridines, alkyl
esters of acrylic and methacrylic acid (e.g., methyl methacrylate, dodecyl
methacrylate, octadecyl methacrylate, etc.), vinyl chloride, vinylidene
chloride, vinyl fluoride, vinylidene fluoride, vinyl esters of carboxylic
acids (e.g., vinyl acetate and vinyl stearate, etc.).
The hydrogenated interpolymers (C) can be either random or block in nature
and many techniques for making both types of interpolymers are well known
to those of skill in the art. See, for example, with relation to random
interpolymers, U.S. Pat. No. 3,752,767, U.S. Pat. No. 2,975,160, U.S. Pat.
No. 3,094,512 and U.S. Pat. No. 3,554,911. With reference to block
polymers, see U.S. Pat. No. 3,333,024, U.S. Pat. No. 3,431,323, U.S. Pat.
No. 3,465,063 and U.S. Pat. No. 3,668,125, as well as German Published
application No. 2,132,336 which corresponds in part to the afore-mentioned
U.S. Pat. No. 3,752,767. Each of these patents is hereby incorporated by
reference for its relevant disclosures.
Briefly, such polymers are usually prepared by anionic polymerization
using, for example, an alkali metal hydrocarbon (e.g., butyl lithium) as a
polymerization catalyst. Other polymerization techniques such as emulsion
polymerization can also be used.
The interpolymers (C) are hydrogenated before use in this invention so as
to remove a substantial portion of their unsaturation, either that derived
from the olefinic groups or from the arene unsaturation, to produce
oil-soluble interpolymers which are useful in the present invention.
Techniques for accomplishing this hydrogenation are well known to those of
skill in the art and need not be described in detail at this point.
Briefly, hydrogenation is accomplished by contacting the interpolymers
with hydrogen at super-atmospheric pressures in the presence of a metal
catalyst such as Raney nickel, palladium supported on charcoal, etc.
Further details of such hydrogenations as well as examples can be found in
the patents mentioned above in relation to the random or block nature of
these interpolymers.
In general, it is preferred that the interpolymers (C), for reasons of
oxidative stability, contain no more than five percent residual olefinic
unsaturation on the basis of the total number of carbon-to-carbon covalent
linkages within the average molecule. Such unsaturation can be measured by
a number of means well-known to those of skill in the art, such as
infrared, NMR, etc. Most preferably, these interpolymers contain no
discernible unsaturation, as determined by the afore-mentioned analytical
techniques.
Specific examples of the interpolymers (C) are the following:
1. Block copolymer completely hydrogenated
(polystyrene-polyisoprene-polystyrene)(35/30/35)w. Number average
molecular weight (M.W.) 1000-2000-1000.
2. Block copolymer completely hydrogenated polystyrene-hydrogenated
polyisoprene-completely hydrogenated polystyrene(1/1/1)m. M.W.
800-1500-800.
3. Block copolymer polyvinylcyclohexane-hydrogenated
polybutadiene-polyvinylcyclohexane(1/1/1)m. M.W. 500-2000-500.
4. Block copolymer polyvinylcyclohexane-ethylene/propylene random
copolymer-polyvinylcyclohexane (1/1/1)m. M.W. 750-1500-750.
5. Block copolymer polyvinylcyclohexane-hydrogenated
polyethylene-polyvinylcyclohexane(1/1/1)m. M.W. 600-1000-600.
6. Block copolymer polyvinylcyclohexane-hydrogenated
polypropylene-polyvinylcyclohexane(1/1/1)m. M.W. 750-1200-750.
7. Block copolymer polyvinylcyclohexane-hydrogenated ethylene/propylene
random copolymer-polyvinylcyclohexane(1/1/1)m. M.W. 700-1100-700.
8. Hydrogenated random butadiene/styrene (35/65)w. M.W. 3000.
9. Hydrogenated random butadiene/styrene (41/59)w. M.W. 4500.
10. Hydrogenated random p-tert-butyl styrene/butadiene(1/1.33)m. M.W. 2500.
11. Hydrogenated random copolymer styrene/isoprene(1/1)m. M.W. 2500.
12. Hydrogenated random copolymer vinyl toluene/butadiene(1/1.33)m. M.W.
3000.
13. Hydrogenated random terpolymer styrene/p-tert-butyl
styrene/butadiene(0.5/0.5/1.33)m. M.W. 3500.
14. Block copolymer completely hydrogenated
[polystyrene-[styrene/butadiene(1/1)m random
copolymer-polystryene](1/1/1)m]. M.W. 800-1500-800.
15. Block copolymer hydrogenated polyisoprene-completely hydrogenated
polystyrene-hydrogenated polyisoprene(1/1/1)m. M.W. 1500-800-1500.
The third component, i.e., component (B), of the lubricant compositions of
this invention, is an oil-soluble extreme pressure agent, or a combination
of two or more such agents. Substances which function as extreme pressure
agents in lubricating oils are of an extremely diverse nature as is well
known to those of skill in the art. See, for example, the particularly
useful discussions in the books "Lubricant Additives" by C. V. Smalheer
and R. Kennedy Smith (Lezius-Hiles Co. Publishers, Cleveland, Ohio, 1967),
pages 9 et seq.; "Gear and Transmission Lubricants", by C. T. Boner
(Reinhold Publishing Corp., New York, 1964), pages 85-100; and "Lubricant
Additives" by M. W. Raney (Noyes Data Corporation, New Jersey, 1973),
pages 146-221. These publications are expressly incorporated herein by
reference for their discussion of how to make and/or use extreme pressure
agents as well as for their discussion of the state of the art with regard
to extreme pressure agents.
Among the types of compounds known by those skilled in the art to be useful
as extreme pressure agents in the lubricating compositions and, therefore,
useful as component (B) of this invention, are organo-boron,
organo-sulfur, organo-halogen, organo-halosulfur, organo-phosphorus,
organo-halophosphorus, and organo-thiophosphorus compounds as well as
esters, amides, and salts of phosphorus and carboxylic acids and
oil-soluble molybdenum-containing complexes such as those disclosed in
U.S. Pat. No. 3,541,014. This patent is hereby incorporated by reference
for its discussion of such complexes. Each of these types of compounds can
be exemplified by the following:
(1) Borate esters of C.sub.1-8 alkanols and adducts prepared by reaction of
succinimides of polyalkylene amines and boric acid. Specific examples
include trioctyl borate and the adduct formed by reaction of boric acid
with a polyisobutenyl succinimide of tetraethylene pentamine.
(2) Sulfurized olefins, sulfurized fatty acid esters of both natural (e.g.,
sperm oil) and synthetic origins, trithiones, thienyl derivatives,
sulfurized terpenes, sulfurized oligomers of C.sub.2-8 monoolefins,
xanthates of alkanols and other organo-hydroxy compounds such as phenols,
thiocarbamates made from alkyl amines and other organo amines, etc.;
sulfurized Diels-Alder adducts such as those disclosed in U.S. Pat. No. RE
27,331. Specific examples include sulfurized polyisobutene of Mn 1100,
sulfurized triisobutene, dicyclohexyl disulfide, diphenyl and dibenzyl
disulfide, etc.
(3) Chlorinated waxes of both the paraffinic and microcrystalline type,
polyhaloaromatics such as di- and trichlorobenzene, trifluoromethyl
naphthalenes, perchlorobenzene, pentachlorophenol, dichloro diphenyl
trichloroethane, etc.
(4) Chlorosulfurized olefins and olefinic waxes, sulfurized chlorophenyl
methyl chlorides, chloroxanthates, etc.; specific examples i | | |
