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Description  |
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This invention relates to liquid hydrocarbon lubricant compositions
comprising copolymers of alpha-olefins and vinyl aromatic compounds and to
their method of preparation. The invention, more particularly, relates to
high viscosity index lubricants that exhibit enhanced thermal stability
and additive solubility, prepared by the copolymerization of
alpha-olefins, or 1-alkenes, and vinyl aromatic monomers using a Group VIB
metal on porous support as catalyst.
BACKGROUND OF THE INVENTION
Recently, novel lubricant compositions (referred to in this specification
as HVI-PAO) produced by oligomerising alpha-olefins and having high values
of viscosity index have been disclosed in U.S. Pat. Nos. 4,827 and
4,827,073, to which reference is made for a description of these
compositions and of their preparation. These materials are produced by
oligomerising the alpha-olefin starting material in the presence of an
oligomerisation catalyst comprising reduced chromium on a silica support
specifically, the oligomers are produced by contacting a C.sub.6 -C.sub.20
1-alkene feedstock with reduced valence state chromium oxide catalyst on
porous silica support under oligomerizing conditions in an oligomerization
zone to produce the desired high viscosity, high viscosity index (VI)
liquid hydrocarbon lubricant. These oligomers are distiguished by having
branch ratios less than 0.19 and the lubricants have notably low pour
points, e.g. pour points below -15.degree. C. Lubricants produced by the
process cover the full range of lubricant viscosities and exhibit a
remarkably high VI and low pour point even at high viscosity. The
as-synthesized HVI-PAO oligomer has olefinic unsaturation associated with
the last of the recurring monomer units in the structure.
Notwithstanding their generally superior properties, HVI-PAO lubricants are
often formulated with additives to enhance those properties for specific
applications. The additives which are more commonly used in lubricants
include oxidation inhibitors, rust inhibitors, metal passivators, antiwear
agents, extreme pressure additives, pour point depressants,
detergent-dispersants, viscosity index improvers, foam inhibitors and the
like. This aspect of the lubricant arts is specifically described in
Kirk-Othmer "Encyclopedia of Chemical Technology", 3rd edition, Vol. 14,
pp.477-526, to which reference is made for a description of such
additives. Improvements in lubricant technology have come both from new
additive development addressed to deficiencies in lubricant basestocks and
new basestocks for inherently better properties.
The inclusion of aromatic compounds in the lubricant mixture is known to
improve thermal stability. Alkylated aromatics, particularly alkylated
naphthalene, are known in the prior art as lubricant additives for their
antiwear properties, thermal and oxidative stability as disclosed in U.S.
Pat. Nos. 4,211,665, 4,238,343, 4,604,491 and 4,714,7944. Antiwear
properties of alkylnaphthalene lubricating fluids are presented in Khimiya
i Tekhnologiya Topliv i Masel, No. 8, pp. 28-29, August, 1986 and show
promise as base stocks for lubricants.
A recurring problem in formulating a new lubricant with an additive package
is the compatibility or solubity of the additive package in the lube, for
specific components of the package may have only very limited solubility
in the aliphatic hydrocarbon lubricant oligomer. This can necessitate the
addition of further additives as solubilizing agents for the package,
adding to the cost and complexity of the lube blend. Consequently, when
the basic structure or backbone of the oligomer can be modified to include
functional groups which confer desirable characteristics on the oligomer
itself, for example, improved thermal stability or solubilizing
characteristics, the foregoing lubricant formulation problems are
mitigated.
SUMMARY OF THE INVENTION
We have now developed HVI-PAO compositions which have improved thermal
stability and additive solubilizing characteristics and which are
extremely useful as liquid lubricants. These lubricants are copolymers or
co-oligomers of alpha-olefins and vinyl aromatic monomers.
To make these liquid lubricants , alpha-olefins, especially lower
alpha-olefins such as 1-decene are copolymerized or co-oligomerised with
vinyl aromatic monomers, especially styrene or the alkyl styrenes such as
methyl styrene, to produce liquid lubricant oligomers having a broad range
of viscosities and high VI. These oligomers also exhibit improved thermal
stability and additive solubilising characteristics. The lubricant
oligomers are random copolymers containing recurring units of 1-alkene and
vinyl aromatic monomer in mole ratios between 2:1 and 500:1, but
preferably between 5:1 and 100:1 and most preferably about 10:1 to 50:1,
e.g.20:1. The recurring 1-alkene units of the copolymer are distinguished
in that they have a branch ratio of less than 0.19, indicative of a poly
1-alkene segment of the copolymer chain or backbone that is essentially
linear.
The catalyst used to prepare these co-oligomers or copolymers comprises is
a reduced Group VIB metal catalyst on porous support, i.e. the same
catalyst system used in the preparation of the HVI-PAO lubricant
oligomers.
In preferred forms, the liquid hydrocarbon lubricant co-oligomers are
prepared from C.sub.6 -C.sub.20 alpha-olefins and vinyl aromatic compounds
by copolymerizing the alpha-olefins and vinyl aromatic compounds with a
Group VIB metal catalyst on a porous support, with the catalyst being
prepared by oxidation at a temperature of 200.degree. C. to 900.degree. C.
in the presence of an oxidizing gas and then by treatment with a reducing
agent at a temperature and for a time sufficient to reduce the metal
component of the catalyst to a lower valence state. The preferred catalyst
comprises reduced chromium oxide on a silica support.
The liquid hydrocarbon lubricant compositions comprise a random copolymer
of C.sub.6 -C.sub.20 alpha-olefin monomer and vinyl aromatic monomer,
wherein the vinyl aromatic monomer has the formula CH.sub.2 .dbd.CH--R
where R is a mono or dinuclear arylene radical, substituted or
unsubstituted, containing 6 to 20 carbon atoms.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention C.sub.6 -C.sub.20 alpha-olefins, or mixtures
thereof, are copolymerized with vinyl aromatic monomers to produce a novel
copolymer. Structurally, the copolymer comprises an addition polymer of
random monomer distribution having the polymeric formula:
##STR1##
where R.sup.1 is an alkyl group containing four to eighteen carbon atoms
and R.sup.2 is an aromatic group comprising a substituted or unsubstituted
arylene radical having six to eighteen carbon atoms. The copolymer has a
ratio of x to y, or mole ratio of different monomer moieties, between 5:1
and 100:1, but preferably from about 10:1 to 50:1.
The copolymer (I) of the instant invention is distinguished over prior art
copolymers of vinyl aromatic monomers and C.sub.6 -C.sub.20 alpha-olefins
in that the copolymer is essentially linear. More particularly, the
segment of the copolymeric backbone comprising recurring units of
alpha-olefin has little isomerization or methyl group branching as a
result of the unique catalyst system used in the process. The branch
ratio, or ratio of methyl to methylene group determined as described
hereinafter, is less than 0.19. Consequently, lubricant grade oligomers of
(I), having an essentially linear structure, exhibit an exceptionally high
viscosity index. Prior art copolymers of alpha-olefins and vinyl aromatic
compounds, to the extent that they contain the high mole fraction of
alpha-olefin required to produce a useful liquid lubricant, contain
isomerized groups in the backbone of the copolymer which produces high
branch ratios and lower values of viscosity index.
Vinyl aromatic monomers useful in the present invention have the formula
CH.sub.2 .dbd.CH--R where R is a mono or dinuclear arylene radical,
substituted or unsubstituted, containing 6 to 20 carbon atoms. Examples of
vinyl aromatic compounds include styrene, alkyl styrenes such as methyl
styrene, ethyl styrene, n-propyl styrene, isopropyl styrene, n-butyl
styrene, tert-butyl styrene, and other vinyl aromatic compounds such as
vinyl biphenyl, 1-vinyl naphthalene, 2-vinyl naphthalene, vinyl-methyl
naphthalene, vinyl-n-hexyl naphthalene. In the case of the alkyl styrenes,
it has been found that both the 3- and 4-alkyl-substituted styrenes will
effectively copolymerise with the alpha-olefins.
Olefins suitable for use as comonomers with vinyl aromatic compounds in the
preparation of the copolymers of the present invention include those
olefins containing from 2 to about 20 carbon atoms such as ethylene,
propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene
and 1-tetradecene and branched chain isomers such as 4-methyl-1-pentene.
Also suitable for use are olefin-containing refinery feedstocks or
effluents. However, the olefins used in this invention are preferably
alpha-olefinic as for example C.sub.6 -C.sub.20 alpha-olefins including
1-hexene to 1-hexadecene and more preferably 1-octene to 1-tetradecene, or
mixtures of such olefins.
The weight ratio of the alpha-olefin to the vinylaromatic compound is
generally in the range of about 99:1 to 2:1, preferably from about 50:1 to
about 10:1.
The catalyst employed for the preparation of the present co-oligomers or
copolymers is a catalyst which is capable of the oligomerizing
alpha-olefins without a significant degree of isomerization to produce the
high viscosity index liquid lubricants referred to above as HVI-PAO
lubricants. Copolymerization of the alpha-olefins with the vinylaromatic
compounds in the presence of the reduced chromium oxide catalysts leads to
a unique copolymer or co-oligomer which is substantially free of double
bond isomerization. Conventional alpha-olefin oligomerization, on the
other hand, promoted by BF.sub.3 or ALC13 forms a carbonium ion which, in
turn, promotes isomerization of the olefinic bond and the formation of
multiple isomers. In the present invention the unique catalyst produces a
liquid hydrocarbon lubricant copolymer with branching ratios less than
0.19.
The branch ratios defined as the ratios of CH.sub.3 groups to CH.sub.2
groups in the lube oil are calculated from the weight fractions of methyl
groups obtained by infrared methods, published in Analytical Chemistry,
Vol. 25, No. 10, p. 1466 (1953).
##EQU1##
The co-monomers used in the process of the present invention are
co-oligomerized or co-polymerised by supported metal oxide catalysts, such
as Cr compounds on silica or other supported IUPAC Periodic Table Group
VIB compounds. The catalyst most preferred is a lower valence Group VIB
metal oxide on an inert support. Preferred supports include silica,
alumina, titania, silica alumina, magnesia aluminum phosphate and the
like. The support material binds the metal oxide catalyst. Those porous
substrates having a pore opening of at least 40 A.degree.. are preferred.
The support material usually has high surface area and large pore volumes
with average pore size of 40 to about 350 angstroms. The high surface area
are beneficial for supporting large amount of highly dispersive, active
chromium metal centers and to give maximum efficiency of metal usage,
resulting in very high activity catalyst. The support should have large
average pore openings of at least 40 A.degree., with an average pore
opening of .noteq.60 to 300 A.degree. preferred. This large pore opening
will not impose any diffusional restriction of the reactant and product to
and away from the active catalytic metal centers, thus further optimizing
the catalyst productivity. Also, for this catalyst to be used in fixed bed
or slurry reactor and to be recycled and regenerated many times, a silica
support with good physical strength is preferred to prevent catalyst
particle attrition or disintegration during handling or reaction.
The supported metal oxide catalyst-..s are preferably prepared by
impregnating metal salts in water or organic solvents onto the support.
Any suitable organic solvent known to the art may be used, for example,
ethanol, methanol, or acetic acid. The solid catalyst precursor is then
dried and calcined at 200 to 900.degree. C. by air or other
oxygen-containing gas. Thereafter the catalyst is reduced by any of
several various and well known reducing agents such as, for example, CO,
H.sub.2, NH.sub.3, H.sub.2 S, CS.sub.2, CH.sub.3 SCH.sub.3, CH.sub.3
SSCH.sub.3, metal alkyl containing compounds such as R.sub.3 Al, R.sub.3
B, R.sub.2 Mg, RLi, R.sub.2 Zn, where R is alkyl, alkoxy, aryl and the
like. Preferred are CO or H.sub.2 or metal alkyl containing compounds.
Alternatively, the Group VIB metal may be applied to the substrate in
reduced form, such as CrII compounds. The resultant catalyst is very
active for copolymerizing alpha-olefins and vinyl aromatic monomers
olefins at a temperature range from below room temperature, eg. as low as
-10.degree. C. , to about 250.degree. C. at a pressure of 0.1 atmosphere
to 5000 psi. A temperature of 25.degree. C. to 250.degree. C. is
preferred. Contact time of the comonomers and the catalyst can vary from
one second to 24 hours. The catalyst can be used in a batch type reactor
or in a fixed bed, continuous-flow reactor.
In general the support material may be added to a solution of the metal
compounds, e.g., acetates or nitrates, etc., and the mixture is then mixed
and dried at room temperature. The dry solid gel is purged at successively
higher temperatures to about 600.degree. C. for a period of about 16 to 20
hours. Thereafter the catalyst is cooled down under an inert atmosphere to
a temperature of about 250 to 450.degree. C. and a stream of pure reducing
agent is contacted therewith for a period when enough CO has passed
through to reduce the catalyst as indicated by a distinct color change
from bright orange to pale blue. Typically, the catalyst is treated with
an amount of CO equivalent to a two-fold stoichiometric excess to reduce
the catalyst to a lower valence CrII state. Finally the catalyst is cooled
down to room temperature and is ready for use.
Examples 1 and 2 below provide an illustration of the preparation of the
catalyst used in the present process.
EXAMPLE 1
Catalyst Preparation and Activation Procedure
1.9 grams of chromium (II) acetate (Cr.sub.2 (OCOCH.sub.3).sub.4 2H.sub.2
O)(5.58 mmole) (commercially obtained) is dissolved in 50 cc of hot acetic
acid. Then 50 grams of a silica gel of 8-12 mesh size, a surface area of
300 m.sup.2 /g, and a pore volume of 1 cc/g, also is added. Most of the
solution is absorbed by the silica gel. The final mixture is mixed for
half an hour on a rotavap at room temperature and dried in an open-dish at
room temperature. First, the dry solid (20 g) is purged with N.sub.2 at
250.degree. C. in a tube furnace. The furnace temperature is then raised
to 400.degree. C. for 2 hours. The temperature is then set at 600.degree.
C. with dry air purging for 16 hours. At this time the catalyst is cooled
down under N to a temperature of 300.degree. C. Then a stream of pure CO
(99.99% from Matheson) is introduced for one hour. Finally, the catalyst
is cooled down to room temperature under N.sub.2 and ready for use.
EXAMPLE 2
A commercial Cr on silica catalyst which contains 1% Cr on a large pore
volume synthetic silica gel is used. The catalyst is first calcined with
air at 700.degree. C. for 16 hours and reduced with CO at 350.degree. C.
for one to two hours.
In carrying out the copolymerization, conditions can be selected to provide
a wide range of viscosities for the liquid lubricant produced. Reactions
carried out at high temperatures produce oligomers with low viscosities
while lower temperature oligomerization reactions produce higher viscosity
product. In either case the product is distinguished by high viscosity
index. Viscosity indices of at least 130 are achieved, ranging up to 280.
Viscosities range from 15cS to 750 cS, measured at 100.degree. C.
The ratio of alpha-olefin to vinyl aromatic monomer in the copolymerization
reaction mixture is selected by considerations of thermal stability and
solubilizing character to be incorporated into the HVI-PAO structure.
Generally, relatively small amounts of vinyl aromatic are sufficient to
significantly improve thermal stability compared to a HVI-PAO lubricant
control. However, the product of this invention can contain between 0.5
and 25 weight percent of the vinyl aromatic moiety, but preferably about
2-10 weight percent.
The copolymerization step is suitably carried out by mixing the monomers
with a small amount of catalyst, generally about 1-5 weight percent, and
heating the reaction mixture in an inert atmosphere. Typical reaction
temperatures are 120-130.degree. C. and reaction times are 1-36 hours. The
catalyst is removed and the reaction mixture product separated to remove
unreacted components. Conventional distillation or similar means are
effective in product separation. The liquid lubricant is recovered in high
yield. The product is then hydrogenated by means well known in the art,
such as over nickel catalyst on kieselguhr, to provide a liquid
hYdrocarbon lubricant of high thermal stability and high viscosity index.
The conditions applicable to the preparation of the copolymer are the same
as those described herein for the preparation of HVI-PAO. Generally, the
reaction is carried out in solution with catalyst in suspension in a
stirred reactor. However, a fixed bed reactor can also be employed. The
product copolymer also exhibits a low pour point of less than -15.degree.
C.
EXAMPLE 3-7
In Examples 3-6 the present copolymerization process and copolymer products
are illustrated and compared with a HVI-PAO homopolymer control, Example
7, prepared under the same conditions.
The catalyst used in all Examples is prepared as described in Example 2.
The copolymerization reaction is carried out as follows: a solution of 100
grams containing styrene in 1-decene is mixed with 5 grams of catalyst and
heated to 130.degree. C. under nitrogen atmosphere. After 16 hours the
catalyst is filtered and the liquid recovered is distilled under vacuum.
Hydrogenation of the recovered liquid is carried out using hydrogen and
nickel catalyst on kieselguhr. Results are presented in Table 1.
TABLE 1
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Hydrogenated Product
Expl.
Wt % Monomer
Lube yield
Viscosity, cS
No. Styrene
1-decene
wt % @ 40.degree. C.
@ 100.degree. C.
VI Pour Pt.
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3 1 99 94.1 355.3
43.2 177
-47.degree. C.
4 3 97 96.1 1019.9
100.6 190
-39.degree. C.
5 4 96 1253.5
110.4 183
6 8 92 37 3164 206.6 185
-24.degree. C.
7 0 100 94 1420 140 214
-40.degree. C.
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The products from the control Example 7, when tested for thermal stability
in comparison with the copolymers prepared in Examples 4 and 5, showed the
results presented in Table 2. The thermal stability tests were carried out
at a temperature of 280.degree. C. under nitrogen atmosphere for 24 hours.
The thermal stability test results clearly show the superior performance of
the copolymer of the present invention over the HVI-PAO homopolymer,
attributable to the inclusion of a relatively small amount of aromatic
component in the backbone of the polymer composition.
TABLE 2
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After Cracking @ 280.degree. C.
Expl. Initial % VcS @
No. VcS @ 100.degree. C.
VI VcS @ 100
VI 100.degree. C. loss
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4 100.6 190 59.8 181 40
5 110.4 183 63.7 174 42
7 145 212 50.75 65
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