Base oils with a high viscosity index and improved low-temperature behavior

A base oil with a high viscosity index and low pour point which is a mixture of a complex ester formed from aliphatic, cycloaliphatic or aromatic dicarboxylic acids, aliphatic polyols containing 2 to 6 hydroxyl groups and aliphatic monocarboxylic acids containing 6 to 22 carbon atoms and adipic acid esters of unbranched aliphatic monohydric alcohols.

FIELD OF THE INVENTION 
This invention relates to base oils with a high viscosity index and 
improved low-temperature behavior which contain complex esters and adipic 
acid esters of unbranched monohydric alcohols, to a process for the 
production of these base oils and to their use as hydraulic, compressor 
and motor oils. 
BACKGROUND OF THE INVENTION 
It has long been known that synthetic esters can be used both as a base oil 
and as an additive in lubricants. By comparison with the less expensive, 
but environmentally less safe mineral oils, synthetic esters were mostly 
used as base oils in cases where the viscosity/temperature behavior was 
expected to meet stringent demands and a low pour point was required, as 
in aircraft turbine oils. Recently, esters based on oleochemicals have 
been acquiring increasing significance because they combine high 
performance levels with biodegradability. Esters based on oleochemicals 
are generally divided into 5 groups, namely: monoesters, glycerol esters, 
dicarboxylic acid esters, polyol esters and complex esters. For 
technological and economic reasons, the dicarboxylic acid esters are 
derived above all from adipic acid, trimethyl adipic acid, sebacic acid, 
azelaic acid, dodecanedioic acid and brassylic acid, C.sub.6-12 
dicarboxylic acid derivatives in particular acquiring increasing 
significance because they show very good viscosity/temperature behavior. 
In the field of lubricating oils, the viscosity/temperature ratio is 
generally characterized by the socalled viscosity index (VI) which is 
determined at 40.degree. and 100.degree. C. in accordance with DIN 51562 
and calculated in accordance with DIN ISO 2909. High VI values indicate 
that minor variations in viscosity are observed at various temperatures. 
Most of the dicarboxylic acid esters mentioned above have high VI values. 
Thus, linear dicarboxylic acid esters of adipic acid in particular, with 
VI values of 200 and higher, show very little dependence of viscosity on 
temperature. However, the disadvantage of the above-mentioned dicarboxylic 
acid esters of adipic acid is their low viscosity so that they are not 
suitable for all fields of application. 
Complex esters made up of polybasic carboxylic acids and polyols and 
monocarboxylic acids show higher viscosities than adipic acid esters. 
Unfortunately, however, these complex esters do not exhibit such good 
viscosity/temperature behavior and, hence, have lower VI values than the 
dicarboxylic acid esters of adipic acid. 
Although both the adipic acid esters and the complex esters show acceptable 
low-temperature behavior insofar as the pour point, i.e. the temperature 
obtained by adding 3.degree. C. to the temperature read off, at which the 
sample no longer flows after cooling, is of the order of -30.degree. C., 
it is of advantage for certain applications for the base oil to have an 
even lower pour point. 
The problem addressed by the present invention was to provide base oils 
which would show improved low temperature behavior (pour point) and which, 
in addition, would have higher viscosities than the known adipic acid 
esters. In addition, these base oils would exhibit very good 
viscosity/temperature behavior and would have VI values above 200. 
SUMMARY OF THE INVENTION 
Surprisingly, base oils of certain complex esters and the dicarboxylic acid 
esters of adipic acid show better low-temperature behavior coupled with a 
high viscosity index. 
The present invention relates to base oils containing 
I. complex esters of 
aliphatic, cycloaliphatic and/or aromatic dicarboxylic acids containing 2 
to 36 carbon atoms, 
b) aliphatic polyols containing 2 to 6 hydroxyl groups, 
c) aliphatic monocarboxylic acids containing 6 to 22 carbon atoms and 
II. adipic acid esters of unbranched aliphatic monohydric alcohols. 
The base oils according to the invention are products liquid at room 
temperature (20.degree. to 25.degree. C.). The complex esters and adipic 
acid esters present in the base oils according to the invention are 
so-called full esters, in other words these esters should not contain any 
free hydroxyl groups or even carboxyl groups per molecule. Instead, the 
complex esters and adipic acid esters should have no hydroxyl or acid 
values or--because esterification is never complete in practice--only low 
hydroxyl or acid values, preferably below 3. 
DETAILED DESCRIPTION OF THE INVENTION 
The adipic acid esters present in the base oils according to the invention 
are compounds known per se which are formed by esterification of adipic 
acid with unbranched aliphatic monohydric alcohols. Adipic acid esters 
derived from saturated alcohols are preferred. Since the adipic acid 
esters should be liquid, those derived from saturated alcohols containing 
1 to 4 carbon atoms, more particularly from methanol, ethanol, propanol 
and/or butanol, are most particularly preferred. Within this group, 
dibutyl adipate is particularly important, especially since it also has an 
excellent VI. 
The complex esters present in the base oils according to the invention are 
also compounds known per se. Thus, DE-C-19 07 768, for example, describes 
the use of such complex esters as lubricants for thermoplastics. These 
complex esters may also readily be prepared by esterification of the 
dicarboxylic acids, polyols and monocarboxylic acids. Suitable aliphatic, 
cycloaliphatic or aromatic dicarboxylic acids are oxalic acid, malonic 
acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic 
acid, azelaic acid, sebacic acid, undecanedicarboxylic acid, 
eicosanedicarboxylic acid, maleic acid, fumaric acid, citraconic acid, 
mesaconic acid, itaconic acid, cyclopropane, cyclobutane and 
cyclopentanedicarboxylic acid, camphoric acid, hexahydrophthalic acid, 
phthalic acid, terephthalic acid, isophthalic acid, naphthalic acid, 
diphenyl-o,o'-dicarboxylic acid and dimer fatty acids. Dimer fatty acids 
in the context of the invention are dicarboxylic acids which are prepared 
by dimerization of mono- or polyunsaturated monocarboxylic acids in the 
presence of catalysts. Preferred dimer fatty acids are those which have 
been prepared by dimerization of monounsaturated C.sub.12-22 
monocarboxylic acids and, in particular, those which have been prepared by 
dimerization of oleic acid. Of the aliphatic dicarboxylic acids, those 
containing 6 to 10 carbon atoms are particularly preferred, the saturated 
representatives thereof being most particularly preferred. Within the 
group of aromatic dicarboxylic acids, phthalic acid is particularly 
suitable. Instead of the dicarboxylic acids, their anhydrides may also be 
used to prepare the complex esters. Overall, aliphatic dicarboxylic acids 
containing 6 to 10 carbon atoms, phthalic acid and/or dimer fatty acid are 
preferred dicarboxylic acids for the complex esters, the dimer fatty acid 
having been prepared by dimerization of monounsaturated C.sub.12-22 
monocarboxylic acids. 
Suitable aliphatic polyols on which the complex esters are based are, for 
example, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 
butane-1,4-diol, butane-2,3-diol, pentane-1,5-diol, hexane-1,6-diol, 
glycerol, trimethylol propane, erythritol, pentaerythritol, 
dipentaerythritol, xylitol, mannitol and/or sorbitol. Particularly 
preferred representatives of these polyols are branched polyols which 
preferably contain tertiary carbon atoms (i.e. those with no hydrogen 
atom) adjacent the primary hydroxyl groups, such as trimethylol propane, 
pentaerythritol, neopentyl glycol, dipentaerythritol and/or mixtures 
thereof. 
Examples of aliphatic C.sub.6-22 monocarboxylic acids from which the 
complex esters are synthesized include caprylic acid, pelargonic acid, 
capric acid, undecanoic acid, lauric acid, lauroleic acid, tridecanoic 
acid, myristic acid, myristoleic acid, pentadecanoic acid, palmitic acid, 
palmitoleic acid, heptadecanoic acid, stearic acid, petroselic acid, 
petroselaidic acid, oleic acid, elaidic acid, linoleic acid, linolaidic 
acid, linolenic acid, nonadecanoic acid, arachic acid, arachidonic acid, 
behenic acid, erucic acid and/or brassidic acid. Among this group of 
saturated and/or unsaturated monocarboxylic acids, the linear 
representatives are preferred. 
The complex esters present in the base oils according to the invention are 
preferably synthesized in such a way that, on a statistical average, 25 to 
75% of the 2 to 6 hydroxyl groups of the polyols are esterified with 
dicarboxylic acids and 75 to 25% with monocarboxylic acids. Calculation of 
the percentage of esterified hydroxyl groups is formally based on the 
number of hydroxyl groups present in the reaction mixture forming the 
complex esters and on the fact that 25 or 75% of this reaction mixture is 
esterified with dicarboxylic acids or monocarboxylic acids. Accordingly, 
so far as the complex esters formed are concerned, it is completely 
immaterial on a statistical average whether the dicarboxylic acid 
esterifies 2 carboxyl groups of one polyol or--in a bridge-like 
arrangement--2 hydroxyl groups of two polyols. 
The base oils according to the invention preferably contain the complex 
esters in quantities of 25 to 75% by weight and the adipic acid esters in 
quantities of 75 to 25% by weight, based on the base oil. 
Particularly preferred base oils contain dibutyl adipate in quantities of 
25 to 75% by weight and complex esters of 
a) dimer fatty acids, adipic acid, pimelic acid, suberic acid, azelaic 
acid, sebacic acid and/or phthalic acid and 
b) neopentyl glycol, trimethylol propane, pentaerythritol, 
dipentaerythritol and/or mixtures thereof and 
c) caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, 
stearic acid and/or oleic acid 
in quantities of 25 to 75% by weight. 
The base oils according to the invention have a pour point which is 
distinctly lower than the pour points of the individual complex esters and 
adipic acid esters. By virtue of their distinctly lower pour point, 
therefore, the low-temperature behavior of the base oils according to the 
invention is very much better because it is only at a relatively low 
temperature that the base oils lose their ability to flow. Accordingly, 
the base oils according to the invention open up fields of application 
which have hitherto been closed to the individual complex esters and 
adipic acid esters. In addition, the base oils according to the invention 
show a distinctly higher viscosity than the pure adipic acid esters, so 
that they are suitable for a broader range of applications than the 
low-viscosity adipic acid esters. In addition, the base oils according to 
the invention show better viscosity/temperature temperature behavior than 
the pure complex esters, so that their range of applications is broader in 
this regard, too. The fact that the complex esters and the adipic acid 
esters can be mixed with one another in broad quantity ratios and still 
always show the improved low-temperature behavior (pour point) is another 
particularly favorable aspect of the base oils according to the invention. 
The base oil can thus be adjusted to any viscosity using the different 
viscosities of the complex esters and the adipic acid ester. Thus, it is 
possible through larger additions of the low-viscosity adipic acid ester 
to obtain an, overall, relatively low-viscosity base oil which, in 
addition, has an extremely high viscosity index. 
The present invention also relates to a process for the production of base 
oils with a high viscosity index and improved low temperature behavior, 
characterized in that complex esters of 
a) aliphatic, cycloaliphatic and/or aromatic dicarboxylic acids containing 
2 to 36 carbon atoms, 
b) aliphatic polyols containing 2 to 6 hydroxyl groups, 
c) aliphatic monocarboxylic acids containing 6 to 22 carbon atoms 
are mixed with adipic acid esters of unbranched, aliphatic monohydric 
alcohols. 
If desired, the base oils according to the invention may additionally 
contain monoesters, glycerol esters and/or polyol esters or even mineral 
oils. The quantity of these additional esters or mineral oils is 
determined by the particular application envisaged. In addition, the base 
oils according to the invention may contain additives for further 
optimization to their particular application. Suitable additives are, for 
example, oxidation inhibitors, such as sulfur, phosphorus, phenol 
derivatives or even amines; detergents, such as naphthenates, stearates, 
sulfonates, phenolates, phosphates, phosphonates or methacrylate 
copolymers; extreme pressure additives, such as sulfur and chlorine 
compounds; foam inhibitors; demulsifiers; corrosion inhibitors and even 
friction coefficient reducers. If desired, known viscosity index 
improvers, such as polyalkyl styrenes, polyolefins, polymethacrylates, 
polyisobutenes and diene polymers, may also be added to the base oils 
according to the invention, although this does make them more sensitive to 
shearing. On the whole, the base oils according to the invention show such 
favorable viscosity/temperature behavior that there is no need to add 
viscosity index improvers. Thus, another advantage of the base oils 
according to the invention is that they are not as sensitive to shearing 
as mineral oils to which viscosity index improvers have to be added. 
The base oils according to the invention may be used for a broad range of 
applications. By virtue of their quality profile, however, they are 
particularly preferred as hydraulic, compressor and motor oils.

Examples 
A) Preparation of the complex esters 
1) Complex ester of trimethylol propane, oleic acid, dimer fatty acid 
171.84 g of trimethylol propane, corresponding to 1.42 moles, 685.2 g of 
technical oleic acid (composition in % by weight: C.sub.16', 5; C.sub.17 
1; C.sub.18 2; C.sub.18' 67; C.sub.18" 12; C.sub.18'" 1; &gt;C.sub.18 2; acid 
value AV according to DIN 53402=198), corresponding to 2.4 moles, and 
327.77 g of C.sub.18 dimer fatty acid with an AV of 193, corresponding to 
1.1 moles, were esterified with one another at a temperature increasing to 
240.degree. C. in the presence of 0.33 g of tin oxalate. The 
esterification was terminated when the acid value of the complex ester 
obtained was below 1. The complex ester was bleached with bleaching clay 
at 90.degree. C. and then filtered off. 
Characteristic data: 
Saponification value SV (according to DIN 53401) 185 
Iodine value IV (according to DGF-C-V 11b) 85 
VI (according to DIN 51562 and calculated in accordance with DIN ISO 2909) 
186 
Pour point (according to DIN ISO 3016) -31.degree. C. 
2) Complex ester of trimethylol propane, oleic acid and dimer fatty acid 
159.05 g of trimethylol propane, corresponding to 1.325 moles, 570 g of 
technical oleic acid (acid value 198), corresponding to 2 moles, and 411.0 
g of C.sub.18 dimer fatty acid (acid value 183), corresponding to 1.3 
moles, were reacted and worked up as in Example 1. A complex ester having 
the following characteristic data was obtained: 
EQU SV=180; IV=85; VI=200; pour point=-32.degree. C. 
3) Complex ester of dipentaerythritol, isononanoic acid, C.sub.8/10 fatty 
acid and phthalic acid 
320.25 g of dipentaerythritol, corresponding to 1.26 moles, 172.5 g of 
isononanoic acid, corresponding to 1.1 moles, 646.5 g of C.sub.8/10 fatty 
acid (composition in % by weight: 69-75 C.sub.8; 23-27 C.sub.10 ; rest 
C.sub.6 and C.sub.12 ; acid value 368), corresponding to 4.2 moles, and 
121.75 g of phthalic anhydride, corresponding to 0.8 mole, were reacted 
and worked up as in Example 1. The complex ester obtained had the 
following characteristic data: SV=360; IV=&lt;1, VI=100; pour 
point=-15.degree. C. 
B.) Production of the base oils 
Complex esters A1) to A3) were mixed with various quantities of dibutyl 
adipate (VI=&gt;250; pour point=-31.degree. C.). The mixing ratios, V.sub.40 
and the pour point of the base oils are shown in Table I. The quantities 
of esters are shown in parts by weight (P). 
TABLE I 
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Base oils and characteristic data 
Base oil Pour point 
Complex ester DBA* V.sub.40 in mm.sup.2 /s 
in .degree.C. 
VI 
__________________________________________________________________________ 
I 61 P Ex. A 1) 
39 P 46.58 -51 255 
II 50 P Ex. A 1) 
50 P 28.68 -51 243 
III 25 P Ex. A 1) 
75 P 10.68 -51 219 
IV 29 P Ex. A 2) 
71 P 19.51 -51 282 
V 43 P Ex. A 3) 
57 P 18.00 -51 215 
Comp. I 
100 
P Ex. A 1) 
0 P 320 -31 186 
Comp. II 
100 
P Ex. A 2) 
0 P 980-1080 
-32 200 
Comp. III 
100 
P Ex. A 3) 
0 P 840 -15 100 
Comp. IV 
0 100 P 
3.6 -31 &gt;250 
__________________________________________________________________________ 
*DBA = dibutyl adipate 
It can be seen from Table I that base oils containing the low-viscosity 
dibutyl adipate and the high-viscosity complex esters on the one hand have 
a higher viscosity than pure dibutyl adipate and, on the other hand, have 
very much lower pour points than the pure starting materials.