Lubricant additive composition for inhibiting viscosity increase and dispersency decrease

A lube oil composition that inhibits lubricant viscosity increase and dispersancy decrease is described. The composition includes an oil of lubricating viscosity and a mixture of specified functionalized derivatives of pentaerythritol tetrakis 3-mercaptopropionate and amine antioxidants.

FIELD OF THE INVENTION 
This invention relates to mixtures of certain pentaerythritol mercapto 
derivatives and amine antioxidants as lubricant additives. 
BACKGROUND OF THE INVENTION 
Antioxidants are added to lube oils to neutralize or minimize oil 
degradation chemistry. For example, U.S. Pat. No. 5,200,101 discloses 
certain amine/hindered phenol, acid anhydride and thiol ester-derived 
products are multi-functional antioxidant, antiwear and rust inhibiting 
lube additives. 
The combination of a metallic dithiophosphate hydroperoxide decomposer and 
aminic antioxidant is reported to have a synergistic effect on lubricant 
antioxidant performance. See Maleville et al, Lubrication Science, V9, No. 
1, pg. 3-60 (1996). Sulfur-substituted derivatives of mercapto carboxylic 
esters also are reported to posses antioxidant properties. See M. A. 
Mirozopeva et al., Naftekhimiya, V28, No. 6, pg. 831-837 (1988). There 
remains a need, nonetheless, for improved lubricant additives and 
lubricant compositions containing them. 
SUMMARY OF THE INVENTION 
It has been found that a mixture of certain functionalized derivatives of 
pentaerythritol tetrakis 3-mercaptopropionate and amine antioxidants 
provide a synergistic effect on inhibiting lubricant viscosity increase 
and dispersancy decrease which would otherwise occur when the lubricants 
are used in an oxidative environment. Thus, in one embodiment there is 
provided a lubricant composition comprising a major portion of an oil of 
lubricating viscosity and a minor portion of an additive comprising a 
mixture of a (i) a derivative of pentaerythritol tetrakis 
3-mercaptopropionate prepared by reacting pentaerythritol tetrakis 
3-mercaptopropionate (PEMP) with maleic anhydride and an alcohol and (ii) 
an amine antioxidant. 
This and other embodiments of the invention will be described in detail 
hereafter.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the invention, to inhibit lubricant viscosity increase 
and dispersancy decrease when a lubricant is used under oxidative 
conditions an effective amount of a mixture of certain functionalized 
derivatives of pentaerythritol tetrakis 3-mercaptopropionate (F-PEMP) and 
amine antioxidants are added to the lubricant. Specifically the F-PEMP 
derivatives are those prepared by reacting PEMP with maleic anhydride (MA) 
and an alcohol (ROH). The mole ratio of PEMP to MA typically will be in 
the range of 1:1 to 1:4 with a mole ratio of 1:2 preferred. The mole ratio 
of MA to ROH typically will be in the range of 1:1 to 1:2 and preferably 
1:1. The alcohols used in preparing the derivatives are selected from 
linear or branched aliphatic primary, secondary or tertiary aliphatic 
alcohols or mixtures thereof having from about 8 to about 28 carbon atoms. 
The reaction is carried out by contacting the PEMP, MA and ROH, in a 
non-reactive solvent such as tetrahydrofuran and preferably in an inert 
atmosphere, at a temperature and for a time sufficient for a F-PEMP to 
form. Typical temperatures are in the range of about 20.degree. C. to 
about 110.degree. C. Times range between about 1 to 24 hours. The F-PEMP 
is readily recovered by vacuum distillation to remove solvent. 
The additive mixture of the present invention also includes an amine 
antioxidant, such as diphenylamines, naphthylphenylamines and alkyl 
substituted derivatives thereof having from about 4 to 24 carbon atoms in 
the alkyl substituent. Particularly preferred is an alkyl 
phenyl-alpha-naphthylamine having from 4 to 12 carbon atoms in the alkyl 
group. 
In the practice the present invention the functionalized derivative 
(F-PEMP) and the amine antioxidant (AO) are used in a ratio sufficient to 
inhibit lubricant viscosity increase and dispersancy decrease. Typically 
F-PEMP to AO ratios will be in the range of 20:80 to 80:20 and preferably, 
30:70 to 50:50 and more preferably 40:60. 
The additive mixtures described herein are utilized in lubricating 
compositions in an amount which will inhibit lubricant viscosity increase 
and dispersancy decrease under conditions of use. Concentrations ranging 
from about 0.1 to 10 wt % based on the total weight of the lubricant 
composition can be used. Preferably the concentration is from 0.5 to 2 wt 
%. 
In general the lubricant used in the compositions of the present invention 
may be any natural or synthetic oil of lubricating viscosity, as for 
example, from about 3 to 20 cSt at 100.degree. C. 
Other additives typically added to lubricants also may be present in the 
composition of the present invention. Such conventional additive types 
include viscosity modifiers, extreme pressure agents, 
corrosion-inhibitors, pour point depressants, detergents, dispersants, 
color stabilizing agents, and other additive materials generally known to 
those skilled in the art. 
The present invention is exemplified by reference to the following 
examples. 
EXAMPLE 1 
This example illustrate the preparation of an F-PEMP. 
Two solutions were prepared, using minimum volumes of tetrahydrofuran; one 
containing PEMP, the other containing maleic anhydride (MA). A sufficient 
amount of each solution was combined at room temperature to provide a 
reaction mixture containing PEMP and MA mole ratio of 1:4. To this 
reaction solution was added 0.02 mol % triethyl amine. The result mixture 
was heated at 60.degree. C. for one hour with stirring and then allowed to 
cool to room temperature. Then n-octanol was added in a mole amount equal 
to the number of moles of MA. Next solvent was removed in vacuum at 
70.degree. C. to yield the F-PEMP, C.sub.65 H.sub.108 O.sub.24 S.sub.4. 
EXAMPLE 2 
A series of compositions were prepared from a 600N base oil containing 5 wt 
% of polyisobutenylsuccinimide and to which was added 1 wt % of the F-PEMP 
of Example 1, an alkyl phenyl-alpha-naphthylamine sold under the trade 
name Irganox L06 by Ciba-Geigy, Basel, Switzerland, or a mixture of the 
two in the weight ratios shown in FIG. 1 in which HDP is the F-PEMP and 
IRG 106 is the Irganox L06. Each sample was evaluated in a 32 hour bench 
oxidation test which was conducted at 165.degree. C. under a mixed 
air/nitrogen flow, with 40 ppm iron from added ferric acetylacetonate as a 
catalyst. The flow rates of air and nitrogen were controlled at 500 
ml/minute and 350 ml/minute, respectively. The samples then were analyzed 
in terms of viscosity changes. The results are given in FIG. 1. 
EXAMPLE 3 
The soot dispersancy of the oil after the oxidation test was determined by 
the viscosity ratio of the diluted test oil in the presence and absence of 
soot; the lower the ratio, the better the dispersancy. The test oil was 
mixed with the soot-laden base oil (3.6 wt % soot) from the GM 6.2 L 
engine at the ratio of 25:75 and the kinematic viscosity at 100.degree. C. 
was measured. At the same time, the kinematic viscosity at 100.degree. C. 
of the test oil--fresh base oil mixture at the same ratio (25:75) was also 
obtained. The results (ratio of these two viscosities) are given in FIG. 
2. As in FIG. 1, HDP is the amount of F-PEMP of Example 1 and IRG L06, the 
amount of Irganox L06 described in Example 2.