Note: Descriptions are shown in the official language in which they were submitted.
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Lubricant composition with an improved viscosity characteristic at low
operating
temperature
The invention relates to lubricant compositions for use in industrial
applications having
improved low temperature viscosity characteristics.
Lubricants for use in industrial applications typically comprise a base oil,
such as a
mineral oil or synthetic oil, and one or more additives. Additives deliver,
for example,
reduced friction and wear, increased viscosity, improved viscosity index, and
resistance
to corrosion, oxidation, aging or contamination.
The ability of a lubricant to reduce friction is dependent on its viscosity.
Generally, the
least viscous fluid, which still forces two moving surfaces apart, is desired.
Many lubricant
applications require good lubricant properties over a broad temperature range,
for
example, when the engine is cold as well as when it has reached its operating
temperature. Therefore, a lubricant's viscosity should change as little as
possible with
temperature, to provide constant lubricant properties over a broad temperature
range.
The temperature-dependence of a lubricant's viscosity is measured by the
viscosity index
(VI). The higher the viscosity index, the smaller is the relative change in
viscosity with
temperature. The viscosity index is determined from the kinematic viscosity at
40 C
(KV40) and the kinematic viscosity at 100 C (KV100), which is a good
reflection of most
engines' operating conditions. Additives which increase the viscosity index
are referred to
as viscosity index improvers (VIls).
Polymers of alkyl (meth)acrylates, and especially polyalkyl(meth)acrylate
based comb
polymers, are known in the art to act as good viscosity index improvers in
lubricant oils.
US patents 5,565,130 and 5,597,871, for example, disclose using comb polymers
comprising polybutadiene-derived macromonomers as viscosity index improvers.
Low
temperature properties are not disclosed therein.
WO 2007/003238 Al describes oil-soluble comb polymers based on polyolefin-
based
macromonomers, especially polybutadiene-based methacrylic esters, and Cl-C10
alkyl
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methacrylates. The comb polymers can be used as an additive for lubricant
oils, in order
to improve the viscosity index and shear stability. They show a particularly
high viscosity
index-improving action in lubricant oils. Low temperature properties are not
disclosed
therein.
The viscosity index, however, does not properly reflect the lubricant's
properties at
temperatures lower than 40 C, for instance at a low temperature range of -20 C
to
+20 C. A lubricant having good lubricant properties at the operating
temperature of a
machine or an engine, therefore, does not necessarily have equally good
properties
during the engine's cold-start phase. The cold-start properties of a lubricant
are, however,
an important factor contributing to an improved fuel efficiency of engines.
The low-temperature properties of lubricants can be measured by the R-factor,
which is
defined as the ratio of the kinematic viscosity at -20 C to the kinematic
viscosity at +20 C.
Since the kinematic viscosity at -20 C is generally higher than at +20 C, the
R-factor is
generally greater than 1. Thus, a narrow viscosity difference between -20 C
and +20 C is
reflected by a low R-factor (an R-factor close to 1).
The problem of providing lubricant compositions having good viscosity
properties at low
operating temperatures has not been sufficiently addressed in the prior art.
Mostly, the
prior art reports the VI measured between 40 C and 100 C, but gives no
indication of the
viscosity characteristics at, for example, -20 C to +20 C.
Therefore, the aim of the present invention is to provide a lubricant
composition with good
low temperature viscosity properties. In particular, the difference of the
kinematic viscosity
of the lubricant composition at -20 C and +20 C should be small. The present
invention
further aims at providing an additive for a lubricant composition for lowering
the difference
between the kinematic viscosity at -20 C and +20 C.
It has been found that the use of certain comb polymers as additives for
lubricant
compositions results in a surprisingly low R-factor that cannot be achieved by
using
conventional viscosity index improvers. It has also been found that lubricant
compositions
comprising a synthetic base oil and the special comb polymers exhibit
surprisingly good
low-temperature viscosity characteristics, in particular a surprisingly good R-
factor.
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The present invention is therefore directed to a lubricant composition,
comprising:
(A) a synthetic base oil; and
(B) a comb polymer, comprising the following monomers:
(a) an ester of
(meth)acrylic acid and a hydroxylated hydrogenated
polybutadiene; and
(b) 0.2%
by weight to 15% by weight of styrene, based on the total
weight of the comb polymer,
wherein the lubricant composition is characterized by an R-factor of 8 or
less, the R-factor
being the ratio of the kinematic viscosity at -20 C to the kinematic viscosity
at +20 C.
The lubricant composition is preferably defined by an R-factor of less than or
equal to 8,
wherein the R-factor is defined as the ratio of the kinematic viscosity of the
lubricant
composition at -20 C (KV_20) to the kinematic viscosity at +20 C (KV+20),
KV_20/KV+20, the
.. kinematic viscosities are measured according to ASTM D445.
Preferably, the lubricant composition has an R-factor of 1 to 8.
The composition is preferably formulated to yield a certain kinematic
viscosity at 40 C
according to ASTM D445. This can be achieved by adjusting the relative amounts
of
comb polymer, base oil and optional additives. Preferably, the composition has
a
kinematic viscosity at 40 C according to ASTM D445 of 10 to 120 mm2/s, more
preferably
40 to 100 mm2/s, most preferably 70 to 80 mm2/s.
In a preferred embodiment, the lubricant composition comprises:
(A) 20 to 90% by weight, preferably 30 to 80% by weight, most preferably 35
to
80% by weight of the synthetic base oil and
(B) 10 to 80% by weight, preferably 20 to 70% by weight, most
preferably 20 to
65% by weight of the comb polymer,
based on the total weight of the lubricant composition.
Suitable synthetic base oils are selected from API Group IV oils. Particularly
preferred are
polyalphaolefins (PA0s).
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The synthetic base oil is typically characterized by its kinematic viscosity,
i.e. the
kinematic viscosity of the pure base oil without any additives. Preferably,
the synthetic
base oil has a kinematic viscosity at 100 C according to ASTM D445 of 1 mm2/s
to 20
mm2/s, more preferably 1 to 10 mm2/s, most preferably 1 to 5 mm2/s and
especially
preferred 2 to 3 mm2/s.
In one embodiment, the synthetic base oil is a polyalphaolefin having a
kinematic
viscosity at 100 C according to ASTM D445 of 1 mm2/s to 20 mm2/s, more
preferably 1 to
mm2/s, most preferably 1 to 5 mm2/s and especially preferred 2 to 3 mm2/s.
A comb polymer in the context of this invention comprises a first polymer,
which is also
referred to as backbone or main chain, and a multitude of further polymers
which are
referred to as side chains and are bonded covalently to the backbone. In the
present
case, the backbone of the comb polymer is formed by the interlinked
unsaturated groups
of the mentioned (meth)acrylates. The ester groups of the (meth)acrylic
esters, the phenyl
radicals of the styrene monomers and the substituents of further free-
radically
polymerizable comonomers form the side chains of the comb polymer. The term
"main
chain" does not necessarily mean that the chain length of the main chain is
greater than
that of the side chains.
One important aspect of the invention is the amount of styrene in the comb
polymer. In
the context of the present invention, the amount of monomers, such as styrene,
is given
in percent by weight based on the total weight of the monomer mixture. Here,
the term
"total weight of the monomer mixture" refers to the total weight of the
monomers,
excluding any additives, such as polymerization initiators, polymerization
promotors,
chain transfer agents and diluents, which might be added to the monomer
mixture to
facilitate polymerization. Provided that all the different monomers present in
the monomer
mixture are equally well incorporated into the copolymer, the relative amounts
of
monomers in the monomer mixture correspond to the relative amounts of the
corresponding monomer units in the copolymer.
In one embodiment, the comb polymer (B) comprises:
(a) at least 20% by weight of esters of (meth)acrylic acid and a
hydroxylated
hydrogenated polybutadiene,
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(b) 0.2% by weight to 15% by weight of styrene, and
(c) optionally further comonomers,
based on the total weight of the comb polymer.
.. The hydroxylated hydrogenated polybutadiene for use in accordance with the
invention
has a number-average molar mass Mn of 4.000 to 6.000 g/mol, preferably 4.000
to 5.000
g/mol. Because of their high molar mass, the hydroxylated hydrogenated
polybutadienes
can also be referred to as macroalcohols in the context of this invention.
The number-average weight Mn is determined by size exclusion chromatography
using
commercially available polybutadiene standards. The determination is effected
to DIN
55672-1 by gel permeation chromatography with THF as eluent.
Preferably, the hydroxylated hydrogenated polybutadiene has a hydrogenation
level of at
least 99%. An alternative measure of the hydrogenation level which can be
determined
on the copolymer of the invention is the iodine number. The iodine number
refers to the
number of grams of iodine which can be added onto 100 g of copolymer.
Preferably, the
copolymer of the invention has an iodine number of not more than 5 g of iodine
per 100 g
of copolymer. The iodine number is determined by the Wijs method according to
DIN
53241-1:1995-05.
Preferred hydroxylated hydrogenated polybutadienes can be obtained according
to GB
2270317.
Some hydroxylated hydrogenated polybutadienes are also commercially available.
The
commercially hydroxylated hydrogenated polybutadienes include, for example,
Kraton
Liquid L-1203, a hydrogenated polybutadiene OH-functionalized to an extent of
about
98% by weight (also called olefin copolymer OCP) having about 50% each of 1,2
repeat
units and 1,4 repeat units, of Mn = 4200 g/mol, from Kraton Polymers GmbH
(Eschborn,
.. Germany). A further supplier of suitable alcohols based on hydrogenated
polybutadiene is
Cray Valley (Paris), a daughter company of Total (Paris), or the Sartomer
Company
(Exton, PA, USA).
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Preference is given to monohydroxylated hydrogenated polybutadienes. More
preferably,
the hydroxylated hydrogenated polybutadiene is a hydroxyethyl- or
hydroxypropyl-
terminated hydrogenated polybutadiene. Particular preference is given to
hydroxypropyl-
terminated polybutadienes.
These monohydroxylated hydrogenated polybutadienes can be prepared by first
converting butadiene monomers by anionic polymerization to polybutadiene.
Subsequently, by reaction of the polybutadiene monomers with ethylene oxide or
propylene oxide, a hydroxy-functionalized polybutadiene can be prepared. This
hydroxylated polybutadiene can be hydrogenated in the presence of a suitable
transition
metal catalyst.
The esters of (meth)acrylic acid for use in accordance with the invention and
a
hydroxylated hydrogenated polybutadiene described are also referred to as
macromonomers in the context of this invention because of their high molar
mass.
The term "(meth)acrylic acid" refers to acrylic acid and methacrylic acid and
to mixtures
thereof; methacrylic acid being particularly preferred. The term
"(meth)acrylate" refers to
esters of acrylic acid and esters of methacrylic acid and to mixtures thereof;
esters of
methacrylic acid being particularly preferred.
The macromonomers for use in accordance with the invention can be prepared by
transesterification of alkyl (meth)acrylates. Reaction of the alkyl
(meth)acrylate with the
hydroxylated hydrogenated polybutadiene forms the ester of the invention.
Preference is
given to using methyl (meth)acrylate or ethyl (meth)acrylate as reactant.
This transesterification is widely known. For example, it is possible for this
purpose to use
a heterogeneous catalyst system, such as lithium hydroxide/calcium oxide
mixture
(Li0H/Ca0), pure lithium hydroxide (Li0H), lithium methoxide (Li0Me) or sodium
methoxide (Na0Me) or a homogeneous catalyst system such as isopropyl titanate
(Ti(OiPr)4) or dioctyltin oxide (Sn(OCt)20). The reaction is an equilibrium
reaction.
Therefore, the low molecular weight alcohol released is typically removed, for
example by
distillation.
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In addition, the macromonomers can be obtained by a direct esterification
proceeding, for
example, from (meth)acrylic acid or (meth)acrylic anhydride, preferably under
acidic
catalysis by p-toluenesulfonic acid or methanesulfonic acid, or from free
methacrylic acid
by the DCC method (dicyclohexylcarbodiimide).
Furthermore, the present hydroxylated hydrogenated polybutadiene can be
converted to
an ester by reaction with an acid chloride such as (meth)acryloyl chloride.
Preferably, in the above-detailed preparations of the esters of the invention,
polymerization inhibitors are used, for example the 4-hydroxy-2,2,6,6-
tetramethylpiperidinooxyl radical and/or hydroquinone monomethyl ether.
Some of the macromonomers for use in accordance with the invention are also
commercially available, for example Kraton Liquid L-1253 which is produced
from
Kraton Liquid L-1203 and is a hydrogenated polybutadiene methacrylate-
functionalized
to an extent of about 96% by weight, having about 50% each of 1,2 repeat units
and 1,4
repeat units, from Kraton Polymers GmbH (Eschborn, Germany). Kraton L-1253 is
likewise synthesized according to GB 2270317.
In addition to the macromonomer and styrene, the monomer mixture may also
comprise
further comonomers, for example other alkyl (meth)acrylates.
Particularly preferred are alkyl (meth)acrylates having 1 to 22 carbon atoms
in the alkyl
chain (also referred to as C1 to C22 alkyl (meth)acrylates).
Suitable alkyl (meth)acrylates are, for example, methyl and ethyl acrylate,
propyl
methacrylate, butyl methacrylate (BMA) and acrylate (BA), isobutyl
methacrylate (IBMA),
hexyl and cyclohexyl methacrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate
(EHA), 2-
ethylhexyl methacrylate, octyl methacrylate, nonyl methacrylate, decyl
methacrylate,
isodecyl methacrylate (IDMA, based on branched (C10)alkyl isomer mixture),
undecyl
methacrylate, dodecyl methacrylate (also known as lauryl methacrylate),
tridecyl
methacrylate, tetradecyl methacrylate (also known as myristyl methacrylate),
pentadecyl
methacrylate, hexadecyl methacrylate (also known as cetyl methacrylate),
heptadecyl
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methacrylate, octadecyl methacrylate (also known as stearyl methacrylate),
nonadecyl
methacrylate, eicosyl methacrylate, behenyl methacrylate and combinations
thereof.
The suitable 010-15 alkyl (meth)acrylates include, for example, decyl
(meth)acrylate,
undecyl (meth)acrylate, 5-methylundecyl (meth)acrylate, dodecyl
(meth)acrylate, 2-
methyldodecyl (meth)acrylate, tridecyl (meth)acrylate, 5-methyltridecyl
(meth)acrylate,
tetradecyl (meth)acrylate and/or pentadecyl (meth)acrylate.
Particularly preferred 010-15 alkyl (meth)acrylates are (meth)acrylic esters
of a linear
012-14 alcohol mixture (012-14 alkyl (meth)acrylate).
In one embodiment, the comb polymer (B) comprises:
(a) 20% by weight to 50% by weight of esters of (meth)acrylic acid
and a
hydroxylated hydrogenated polybutadiene,
(b) 0.2% by weight to 15% by weight of styrene, and
(c) up to 70% by weight of alkyl (meth)acrylates having 1 to 22 carbon
atoms
in the alkyl chain,
based on the total weight of the comb polymer.
In one embodiment, the comb polymer comprises:
(a) 20% by weight to 35% by weight of esters of (meth)acrylic acid and a
hydroxylated hydrogenated polybutadiene,
(b) 0.2% by weight to 15% by weight of styrene,
(c) 0.2% by weight to 20% by weight of alkyl (meth)acrylates having 12 to
15
carbon atoms in the alkyl chain, and
(d) 50% by weight to 70% by weight of alkyl (meth)acrylates having 1 to 4
carbon atoms in the alkyl chain,
based on the total weight of the comb polymer.
In one embodiment, the comb polymer (B) comprises:
(a) 25% by weight to 30%by weight of esters of (meth)acrylic acid and a
hydroxylated hydrogenated polybutadiene,
(b) 0.2% by weight to 15% by weight of styrene,
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(c) 0.2% by weight to 15% by weight of alkyl methacrylates having 12 to 14
carbon atoms in the alkyl chain,
(d) 50% by weight to 65% by weight of butylmethacrylate, and
(e) 0.1% by weight to 0.2% by weight of methylmethacrylate,
based on the total weight of the comb polymer.
The polyalkyl(meth)acrylate based comb polymer in accordance with the
invention may
preferably be obtained by radical polymerization. However, the comb polymer
may also
be obtained by polymer-analogous reactions and/or graft copolymerization.
In one embodiment, the comb polymer may be obtained by radical polymerization
involving the steps of
a) providing a monomer mixture comprising the indicated monomers; and
b) initiating radical polymerization in the monomer mixture.
The polymerization reaction is preferably initiated by mixing the monomer
mixture with a
radical initiator. In some cases, it may be required to heat the reaction
mixture to the
reaction temperatures specified below to initiate the polymerization.
.. Radical initiators may be selected from any of the well-known free-radical-
producing
compounds such as peroxy, hydroperoxy and azo initiators, including, for
example, acetyl
peroxide, benzoyl peroxide, lauroyl peroxide, tert-butyl peroxyisobutyrate,
caproyl
peroxide, cumene hydroperoxide, 1, 1-di(tert-butylperoxy)-3,3,5-
trimethylcyclohexane,
azobisisobutyronitrile and tert-butyl peroctoate (also known as tert-
butylperoxy-2-
ethylhexanoate). Preferable radical initiators are benzoyl peroxide, lauroyl
peroxide, tert-
butyl peroxyisobutyrate, azobisisobutyronitrile and tert-butyl peroctoate.
Tert-butyl
peroctoate is particularly preferred. The initiator concentration is typically
between 0.025
and 1 wt-%, preferably from 0.05 to 0.75 wt-%, more preferably from 0.1 to 0.5
wt-% and
most preferably from 0.2 to 0.4 wt-%, based on the total weight of the
monomers.
The polymerization is preferably conducted at a temperature below the boiling
point of the
reaction mixtures. Preferably, the temperature is in the range of 60 to 150 C,
more
preferably 85 to 130 C, most preferably 90 to 110 C.
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One or more polymerization promoters may also be added to the monomer mixture.
Suitable promoters include, for example, quaternary ammonium salts such as
benzyl(hydrogenated-tallow)-dimethylammonium chloride and amines. Preferably
the
promoters are soluble in hydrocarbons. When used, these promoters are present
at
levels from 1 to 50 wt-%, preferably from 5 to 25 wt-%, based on total weight
of initiator.
Chain transfer agents may also be added to control the molecular weight of the
copolymer. The preferred chain transfer agents are alkyl mercaptans such as
lauryl
mercaptan (also known as dodecyl mercaptan, DDM). The amount of chain transfer
agent
is preferably 5 wt-% or less, more preferably 2 wt-% or less, based on the
total weight of
monomers.
Diluents may also be added to the monomer mixture. Preferably, the first and
second
reaction mixture each comprise up to 60 wt-% diluent, more preferably 5 to 60
wt-%, most
preferably 10 to 60 wt-%.
Among the diluents suitable for use in the process of the present invention
for non-
aqueous solution polymerizations are aromatic hydrocarbons (such as benzene,
toluene,
xylene and aromatic naphthas), chlorinated hydrocarbons (such as ethylene
dichloride,
chlorobenzene and dichlorobenzene), esters (such as ethyl propionate or butyl
acetate),
(C6-C20)aliphatic hydrocarbons (such as cyclohexane, heptane and octane),
mineral oils
(such as paraffinic and naphthenic oils) or synthetic base oils (such as
poly([alphaFolefin)
oligomer (PAO) lubricating oils, for example, [alpha]-decene dimers, trimers
and mixtures
thereof).
The lubricant composition in accordance with the present invention may further
comprise
auxiliary additives selected from the group consisting of pour point
depressants, antiwear
agents, antioxidants, dispersants, detergents, friction modifiers, antifoam
agents, extreme
pressure additives, and corrosion inhibitors. The auxiliary additives are
preferably added
in an amount of 0.1 to 25 weight-%, based on the total weight of the lubricant
composition.
Suitable pour-point depressants include ethylene-vinyl acetate copolymers,
chlorinated
paraffin-naphthalene condensates, chlorinated paraffin-phenol condensates,
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polymethacrylates, polyalkylstyrenes, etc. Preferred are polymethacrylates
having a
mass-average molecular weight of from 5.000 to 50.000 g/mol.
The preferred antiwear and extreme pressure additives include sulfur-
containing
compounds such as zinc dithiophosphate, zinc di-03-12-alkyldithiophosphates
(ZnDTPs),
zinc phosphate, zinc dithiocarbamate, molybdenum dithiocarbamate, molybdenum
dithiophosphate, disulfides, sulfurized olefins, sulfurized oils and fats,
sulfurized esters,
thiocarbonates, thiocarbamates, polysulfides, etc.; phosphorus-containing
compounds
such as phosphites, phosphates, for example trialkyl phosphates, triaryl
phosphates, e.g.
tricresyl phosphate, amine-neutralized mono- and dialkyl phosphates,
ethoxylated mono-
and dialkyl phosphates,phosphonates, phosphines, amine salts or metal salts of
those
compounds, etc.; sulfur and phosphorus-containing anti-wear agents such as
thiophosphites, thiophosphates, thiophosphonates, amine salts or metal salts
of those
compounds, etc..
The suitable antioxidants include, for example, phenol-based antioxidants and
amine-
based antioxidants.
Phenol-based antioxidants include, for example, octadecy1-3-(3,5-di-tert-butyl-
4-
hydroxyphenyl)propionate; 4,4' -methylenebis(2,6-di-tert-butylphenol); 4,4' -
bis(2,6-di-t-
butylphenol); 4,4' -b is(2-methyl-6-t-butylphenol); 2,2' -methylenebis(4-ethyl-
6-t-
butylphenol); 2,2' -methylenebis( 4-methyl-6-t-butyl phenol); 4,4' -butyl
idenebis(3-methyl-
6-t-butylphenol); 4,4'-isopropylidenebis(2,6-di-t-butylphenol); 2,2'-
methylenebis(4-methyl-
6-nonylphenol); 2,2'-isobutylidenebis(4,6-dimethylphenol); 2,2'-methylenebis(4-
methyl-6-
cyclohexylphenol); 2,6-di-t-butyl-4-methylphenol; 2,6-di-t-butyl-4-ethyl-
phenol; 2,4-
dimethy1-6-t-butylphenol; 2,6-di-t-amyl-p-cresol; 2,6-di-t-butyi-4-(N,N'-
dimethylaminomethylphenol); 4,41thiobis(2-methyl-6-t-butylphenol); 4,4'-
thiobis(3-methyl-
6-t-butylphenol); 2,2'-thiobis(4-methyl-6-t-butylphenol); bis(3-methyl-4-
hydroxy-5-t-
butylbenzyl) sulfide; bis(3,5-di-t-butyl-4-hydroxybenzyl) sulfide; n-octy1-3-
(4-hydroxy-3,5-
di-t-butylphenyl)propionate; n-octadecy1-3-(4-hydroxy-3,5-di-t-
butylphenyl)propionate;
2,2'-thio[diethyl-bis-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], etc. Of
those, especially
preferred are bis-phenol-based antioxidants and ester group containing phenol-
based
antioxidants.
The amine-based antioxidants include, for example, monoalkyldiphenylamines
such as
monooctyldiphenylamine, monononyldiphenylamine, etc.; dialkyldiphenylamines
such as
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4,4' -dibutyldiphenylamine, 4,4'-dipentyldiphe nylamine, 4,4'-
dihexyldiphenylamine, 4,4'-
diheptyldiphenylamine, 4,4'-dioctyldiphenylamine, 4,4'-dinonyldiphenylamine,
etc.;
polyalkyldiphenylamines such as tetrabutyldiphenylamine,
tetrahexyldiphenylamine,
tetraoctyldiphenylamine, tetranonyldiphenylamine, etc.; naphthylamines,
concretely
alpha-naphthylamine, phenyl-alpha-naphthylamine and further alkyl-substituted
phenyl-
alpha-naphthylamines such as butylphenyl-alpha-naphthylamine, pentylphenyl-
alpha-
naphthylamine, hexylphenyl-alpha-naphthylamine, heptylphenyl-alpha-
naphthylamine,
octylphenyl-alpha-naphthylamine, nonylphenyl-alpha-naphthylamine, etc. Of
those,
diphenylamines are preferred to naphthylamines, from the viewpoint of the
antioxidation
effect thereof.
Suitable antioxidants may further be selected from the group consisting of
compounds
containing sulfur and phosphorus, for example metal dithiophosphates, for
example zinc
dithiophosphates (ZnDTPs), "OOS triesters" = reaction products of
dithiophosphoric acid
with activated double bonds from olefins, cyclopentadiene, norbornadiene, a-
pinene,
polybutene, acrylic esters, maleic esters (ashless on combustion);
organosulfur
compounds, for example dialkyl sulfides, diaryl sulfides, polysulfides,
modified thiols,
thiophene derivatives, xanthates, thioglycols, thioaldehydes, sulfur-
containing carboxylic
acids; heterocyclic sulfur/nitrogen compounds, especially
dialkyldimercaptothiadiazoles,
2-mercaptobenzimidazoles; zinc bis(dialkyldithiocarbamate) and methylene
bis(dialkyldithiocarbamate); organophosphorus compounds, for example triaryl
and
trialkyl phosphites; organocopper compounds and overbased calcium- and
magnesium-
based phenoxides and salicylates.
Appropriate dispersants include poly(isobutylene) derivatives, for example
poly(isobutylene)succinimides (PIBSIs), including borated PIBSIs; and ethylene-
propylene oligomers having N/O functionalities.
The preferred detergents include metal-containing compounds, for example
phenoxides;
salicylates; thiophosphonates, especially thiopyrophosphonates,
thiophosphonates and
phosphonates; sulfonates and carbonates. As metal, these compounds may contain
especially calcium, magnesium and barium. These compounds may preferably be
used in
neutral or overbased form.
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Friction modifiers used may include mechanically active compounds, for example
molybdenum disulfide, graphite (including fluorinated graphite),
poly(trifluoroethylene),
polyamide, polyimide; compounds that form adsorption layers, for example long-
chain
carboxylic acids, fatty acid esters, ethers, alcohols, amines, amides, imides;
compounds
which form layers through tribochemical reactions, for example saturated fatty
acids,
phosphoric acid and thiophosphoric esters, xanthogenates, sulfurized fatty
acids;
compounds that form polymer-like layers, for example ethoxylated dicarboxylic
partial
esters, dialkyl phthalates, methacrylates, unsaturated fatty acids, sulfurized
olefins or
organometallic compounds, for example molybdenum compounds (molybdenum
dithiophosphates and molybdenum dithiocarbamates MoDTCs) and combinations
thereof
with ZnDTPs, copper-containing organic compounds.
Suitable antifoam agents are silicone oils, fluorosilicone oils, fluoroalkyl
ethers, etc..
The above-detailed additives are described in detail, inter alia, in T. Mang,
W. Dresel
(eds.): "Lubricants and Lubrication", Wiley-VCH, Weinheim 2001; R. M. Mortier,
S. T.
Orszulik (eds.): "Chemistry and Technology of Lubricants".
The lubricant composition according to the invention can be useful for various
applications including industrial gear oil, lubricant for wind turbine,
compressor oil,
hydraulic fluid, paper machine lubricant, engine or motor oil, transmission
and/or drive-
trains fluid, machine tools lubricant, metalworking fluids, and transformer
oils to name a
few.
In a further aspect, the invention also relates to the use of a comb polymer
as described
above as an additive for a base oil to produce a lubricant composition,
characterized in
that the lubricant composition preferably has an R-factor of less than or
equal to 8,
wherein the R-factor is defined as the ratio of the kinematic viscosity of the
lubricant
composition at -20 C to the kinematic viscosity at +20 C measured according to
ASTM
D445. Preferably, the lubricant composition has an R-factor of 1 to 8.
Preferably, the lubricant composition comprises a synthetic base oil. The base
oil is
preferably selected form the group consisting of polyalphaolefins, naphthenic
base oils
and mixtures thereof. More preferably, the lubricant composition comprises a
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polyalphaolefin base oil having a kinematic viscosity at 100 C according to
ASTM D445
of 1 mm2/s to 20 mm2/s, more preferably 1 to 10 mm2/s, most preferably 1 to 5
mm2/s and
especially preferred 2 to 3 mm2/s.
The lubricant composition preferably has a kinematic viscosity at 40 C
according to
ASTM D445 of 10 to 120 mm2/s, more preferably 40 to 100 mm2/s, most preferably
70 to
80 mm2/s.
In a further aspect the invention relates to a method of producing a lubricant
composition
having an R-factor of less than or equal to 8, preferably 1 to 8, wherein the
R-factor is
defined as the ratio of the kinematic viscosity of the lubricant composition
at -20 C to the
kinematic viscosity at +20 C measured according to ASTM D445, the method
comprising
the step of adding a comb polymer according to the present invention to a
synthetic base
oil, the synthetic base oil being preferably a polyalphaolefin base oil having
a kinematic
viscosity at 100 C according to ASTM D445 of 1 mm2/s to 20 mm2/s, more
preferably 1 to
10 mm2/s, most preferably 1 to 5 mm2/s and especially preferred 2 to 3 mm2/s.
The method is preferably characterized by adding the comb polymer in an amount
to get
a lubricant composition with a kinematic viscosity at 40 C according to ASTM
D445 of 10
to 120 mm2/s, more preferably 40 to 100 mm2/s, most preferably 70 to 80 mm2/s.
Experimental Part
Synthesis of a hydroxylated hydrogenated polybutadiene
The macroalcohol prepared was a hydroxypropyl-terminated hydrogenated
polybutadiene
having a mean molar mass Mn = 4750 g/mol.
The macroalcohol was synthesized by an anionic polymerization of 1,3-butadiene
with
butyllithium at 20-45 C. On attainment of the desired degree of
polymerization, the
reaction was stopped by adding propylene oxide and lithium was removed by
precipitation with methanol. Subsequently, the polymer was hydrogenated under
a
hydrogen atmosphere in the presence of a noble metal catalyst at up to 140 C
and
pressure 200 bar. After the hydrogenation had ended, the noble metal catalyst
was
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removed and organic solvent was drawn off under reduced pressure. Finally, the
base oil
NB 3020 was used for dilution to a polymer content of 70% by weight.
The vinyl content of the macroalcohol was 61%, the hydrogenation level >99%
and the
OH functionality > 98%. These values were determined by H-NMR (nuclear
resonance
spectroscopy).
Synthesis of macromonomer (MM)
In a 2 L stirred apparatus equipped with saber stirrer, air inlet tube,
thermocouple with
controller, heating mantle, column having a random packing of 3 mm wire
spirals, vapor
divider, top thermometer, reflux condenser and substrate cooler, 1000 g of the
above-
described macroalcohol are dissolved in 450 g of methyl methacrylate (MMA) by
stirring
at 60 C. Added to the solution are 20 ppm of 2,2,6,6-tetramethylpiperidin-1-
oxyl radical
and 200 ppm of hydroquinone monomethyl ether. After heating to MMA reflux
(bottom
temperature about 110 C) while passing air through for stabilization, about 20
g of MMA
are distilled off for azeotropic drying. After cooling to 95 C, 0.30 g of
LiOCH3 is added
and the mixture is heated back to reflux. After the reaction time of about 1
hour, the top
temperature has fallen to ¨64 C because of methanol formation. The
methanol/MMA
azeotrope formed is distilled off constantly until a constant top temperature
of about
100 C is established again. At this temperature, the mixture is left to react
for a further
hour. For further workup, the bulk of MMA is drawn off under reduced pressure.
Insoluble
catalyst residues are removed by pressure filtration (Seitz T1000 depth
filter). The content
of NB 3020 "entrained" into the copolymer syntheses described further down was
taken
into account accordingly.
Synthesis of comb polymers
Comb polymers according to the invention were prepared according to the
following free
radical polymerization procedure.
In a glass beaker, a mixture of 100 g of all the monomers (e.g. as detailed in
Table 1) is
diluted in ester oil (e.g. diisononyl adipate) to reach approximately 60% w/w
dilution at
90 C. Afterwards, 35% of this diluted mixture is charged into a continuously
stirred glass
reactor, followed by addition of 0.105 g of initiator t-butylperoxy 2-
ethylhexanoate. The
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rest of the monomer mixture is gradually added into the glass reactor at
constant flow rate
for 3 h, in parallel with the addition of another 0.195 g of initiator t-
butylperoxy 2-
ethylhexanoate, also being introduced at constant flow rate for 3 h. Reaction
temperature
is held constant at 90 C. After 3 hours, additional 2x0.2 g of initiator t-
butylperoxy 2-
ethylhexanoate is added to ensure the completion of the polymerization, 2 and
5 hours
after the end of the monomer feeding, while the reaction is kept at 90 C. At
the end of the
reaction, additional dilution ester oil can be added.
Lauryl methacrylate is a mixture of linear 012 and C14 alkyl methacrylates
with a ratio of
C12 to C14 alkyl methacrylate of around 73/27.
The compositions of the monomer mixtures used to prepare exemplary copolymers
according to the invention are given in the following Table 1. The amounts of
monomers
are given as weight-% based on the total weight of the comb polymer.
Table 1: Net compositions of the comb polymers prepared to support the
present
invention
comb polymer A B C
(inventive) (inventive) (CE)
macromonomer 30% 25% 42%
lauryl methacrylate 15% 0.2% 0.2%
(C12 to C14 methacrylate)
methyl methacrylate 0.2% 0.2% 0.2%
n-butyl methacrylate 54.6% 63.55% 17.8%
styrene 0.2% 11.05% 39.8%
Total 100 100 100
CE = comparative example
Comb polymers A and B are inventive examples and have a styrene content
claimed by
the present invention. Comb polymer C is a comparative example and comprises a
higher
amount of styrene. It was prepared to show that a higher amount of styrene
leads to a
higher R-factor.
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The following polyalkylmethacrylates are known viscosity index improvers and
were used
in comparative lubricant compositions. They were prepared to show that an R-
factor
within the claimed range of 1-8 can only be reached by using comb polymers
which
comprise a certain amount of an ester of (meth)acrylic acid and a hydroxylated
hydrogenated polybutadiene.
Comparative copolymer D is a polyalkylmethacrylate prepared through similar
aforementioned free radical polymerization procedure from 89.97% by weight of
dodecyl
pentadecyl methacrylate and 10.03% by weight of methyl methacrylate.
Dodecyl pentadecyl methacrylate is a mixture of branched and linear C12 to C15
alkyl
methacrylates with an average composition of 16 to 26% by weight of C12 alkyl
methacrylate, 24 to 34% by weight of C13 alkyl methacrylate, 24 to 34% by
weight of C14
alkyl methacrylate, and 16 to 26% by weight of C15 alkyl methacrylate, and
approximately
80% linear alkyl methacrylates.
Comparative copolymer E is a polyalkylmethacrylate prepared through similar
aforementioned free radical polymerization procedure from 99.80% by weight of
C12 to
C15 alkyl methacrylate (comprising 20% C12 alkyl methacrylate, 34% C13 alkyl
methacrylate, 29% C14 alkyl methacrylate, and 17% C15 alkyl methacrylate, with
approximately 40% linear alkyl methacrylates) and 0.20% by weight of methyl
methacrylate.
Lubricant compositions were obtained by mixing a poly-alpha-olefin base oil
(PA02)
having a kinematic viscosity at 100 C of 2 mm2/s according to ASTM D445 and
the
indicated amount of copolymers as viscosity index improvers according to the
following
table targeting a KV40 of 76 mm2/s.
Table 2: lubricant compositions comprising comb polymers and base oil
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No Base oil Copolymer Amount of styrene Amount of copolymer
Pk by weighty) Pk by weighty*)
1 PA02 A 0.2 25.2
2 PA02 B 11.0 63.9
3 PA02 C 39.8 40.8 CE
4 PA02 D -- 41.1 CE
PA02 E -- 48.5 CE
*) based on the total weight of the comb polymer
**) based on the total weight of the lubricant composition
CE: comparative example
5 The lubricant compositions were tested by determining the kinematic
viscosity at different
temperatures, the viscosity index and the R-factor. The kinematic viscosity
was
determined according to ASTM D445, the viscosity index was determined
according to
ASTM D2270, and the R-factor was calculated as the ratio of the kinematic
viscosity
at -20 C to the kinematic viscosity at +20 C. The results are given in the
following Table
3.
Table 3: viscosity data of lubricant compositions
No. Kinematic viscosity (mm2/s) VI R-factor
100 C 40 C 20 C 10 C 0 C -10 C -20 C
1 24.36 75.76 140.2 191.8 264.1 381.1 635.7 345 4.53
2 102.1 76.40 130.2 193.0 319.9 597.2 877.4 810 6.74
3 27.01 76.59 140.8 215.1 362.7 674.4 1387 373 9.85 CE
4 17.55 76.08 165.0 264.3 450.8 825.2 1686 251 10.22 CE
5 14.83 76.39 180.2 304.5 550.5 1491 2375 205 13.18 CE
CE: comparative example
The data demonstrate that the lubricant compositions according to the
invention
(compositions 1 and 2, comprising the comb polymers A and B, respectively)
have a high
viscosity index as well as a low R-factor. The compositions according to the
invention,
therefore, guarantee a good viscosity characteristic at low operating
temperature, defined
as an R-factor lower than 8. In contrast, the comparative lubricant
compositions
(compositions 3, 4 and 5, comprising comb polymers with a higher amount of
styrene or
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conventional polyalkylmethacrylates D and E) exhibit a good viscosity index
but do not
perform as well at low operating temperatures. They show R-factors greater
than 8.
The data, therefore, show that the use of comb polymers according to the
present
invention has the surprising advantage over the use of conventional viscosity
index
improvers of improving the viscosity characteristics of a lubricant not only
at normal
operating temperatures but also at low operating temperatures.
When formulated to a given KV40 of 76 mm2/s, the data of Table 3 show that
with
decreasing temperature the kinematic viscosity of the formulations is
increasing.
By using the inventive polymers (Examples 1 and 2) the viscosity increase is
much lower
compared to the use of standard polymers (Examples 4 and 5) or a polymer with
higher
styrene content (Example 3).
When formulated to a given KV40 of 76 mm2/s, the lubricant compositions
comprising the
comb polymers according to the present invention show a KV20 in the range of
130 to 140
and a KV_20 in the range of 640 to 870.
It is further shown that the definition of VI is not appropriate to
extrapolate viscosities
down to temperatures below 40 C, e.g. to -20 C.
