Solid anti-friction devices – materials therefor – lubricant or se – Lubricants or separants for moving solid surfaces and... – Organic -co- compound
Reexamination Certificate
1999-07-21
2002-02-12
Medley, Margaret (Department: 1721)
Solid anti-friction devices, materials therefor, lubricant or se
Lubricants or separants for moving solid surfaces and...
Organic -co- compound
C554S001000
Reexamination Certificate
active
06346504
ABSTRACT:
The present invention relates to ester based lubricants for four-stroke engines, which comprise at least one ester containing one ester bond.
Friction is the force which resists the relative motion of two contacting bodies and in practice several distinct friction regimes are distinguished. If sliding surfaces are in direct contact there is dry or solid friction, but if the sliding surfaces are separated by a solid, fluid or gaseous medium, then there is lubricated or fluid friction. In the regime of the transition from lubricated to dry friction one has mixed friction in which regime both former types of friction occur simultaneously. The mixed friction in the regime of transition to dry friction is called boundary friction. The total picture is therefore: dry friction—boundary friction—mixed friction—fluid friction. The function of a lubricant is to reduce the friction between two contacting and moving bodies as much as possible and thus to prevent wear. The lubricant will also remove the heat of friction and the wear particles from the load-carrying zone and it will seal off the lubricated area so that nothing can enter this zone which might disrupt the lubrication action.
The relation between viscosity (&ngr;), bearing load (p), sliding velocity (v) and friction coefficient (&mgr;) has been depicted by R. Stribeck (1902) in a curve which since then is called the Stribeck curve. This curve is obtained by plotting the friction coefficient &mgr; along the ordinate and the product {fraction (xv/p)}
along the absciss.
In the first (left hand) part of this graph the curve falls rather steeply and has the form of half a parabola, but at a certain moment it shows a point of inflection, after which the curve is gradually but slowly climbing again. The point of inflection in the Stribeck curve occurs at the so-called transition speed where mixed friction passes into fluid friction. The working range of the lubricant is then defined by the lower and upper working limits both situated in the right hand part of the Stribeck curve where fluid friction is fully developed. The lower working limit is situated as close as possible to the point of inflection. The Stribeck curve shows that the properties of the lubricant (particularly its viscosity) are to be selected such that the best compromise is reached between friction losses in the region of hydrodynamic lubrication (fully developed film) and bearing wear in the region of mixed friction. Thus the lower working limit is selected preferably as close as possible to the point of inflection of the transition speed, but the closer one is to this point, the greater will be the influence of the additives on the lubricant, in other words: the selection of the additives will be very critical at the lower working limit. From the Stribeck curve it can be concluded that for a given situation of bearing load and sliding velocity, the performance of the lubricant will to a large extent be dictated by its viscosity.
The viscosity of ester based lubricants is not only dependent on their molecular weight, but also and particularly on their molecular structure and the presence of unreacted hydroxyl groups. The requirements for a good viscosity are often conflicting, however, with the molecular and structural requirements for good flow properties (viscosity index (V.I.) and pour point), good lubricity (polarity) and thermal and oxidative stability. Thus improvement of certain properties is not seldom achieved at the cost of other properties.
It has now been found in extensive experiments that excellent ester based lubricants having a set of good properties may be obtained by a careful selection of their chemical and molecular structure. These esters have only one ester bond and hence constitute simple esters, contrary to the complex esters which are often used in ester based lubricants. It is surprising that such relatively simple molecules exhibit various good properties at the same time and this the more so since no polyhydric alcohols and/or polybasic acids are used in their manufacture, thus restricting the amount of possibilities in achieving certain desired chemical structures and physical properties.
Since the esters have only one ester bond, their polarity due to the lone pair on the oxygen atom of the ester linkage is relatively low in comparison to the polyhydric alcohol based esters and the complex esters. Polar molecules are very effective boundary lubricants, however, since they tend to form physical bonds with the metal surface. It is therefore surprising that the presence of only one ester bond can still provide sufficient lubricity. At the same time the efficiency of antiwear additives is still high. A problem with very polar base fluids is that these preferentially cover the metal surface instead of the antiwear additives and consequently there is higher wear. Stated differently: there is competition between the ester lubricant and the antiwear additives. The ester based lubricants according to the present invention, which are particularly suitable for use in four-stroke engines, enable an efficient use of the various additives with optimum effect and at the same time have sufficiently low viscosity for a good fuel economy of the lubricated engine, whilst yet retaining good flow properties and lubricity and a low volatility (important for longer oil-change intervals).
Therefore the present invention relates to an ester based lubricant for four-stroke engines comprising at least one ester of a saturated, branched chain aliphatic monohydric alcohol having at least 8 carbon atoms and a saturated, branched chain aliphatic monocarboxylic acid having at least 10 carbon atoms, said ester having:
(a) a kinematic viscosity at 40° C. of at most 35 cSt,
(b) a non-polarity index (NPI)
NPI
=
total
number
of
carbon
atoms
×
molecular
weight
number
of
carboxylate
groups
×
100
of
least
at
100
,
of at least 100,
(c) an evaporation loss according to Noack (determined according to European Standard CEC L-40-T-82) of at most 10%, and
(d) a pour point below −30° C.
The ester based lubricants according to the present invention may be based on one single ester, but also mixtures of esters may be used. The use of mixtures of esters according to the present invention may sometimes lead to positive synergism in required properties, for example the pour point may be improved. The use of ester mixtures is therefore preferred. Also the esters according to the present invention may be mixed with other simple esters.
The saturated, branched chain aliphatic monohydric alcohols are preferably selected from the group consisting of Guerbet alcohols, oxo alcohols, aldol condensation derived alcohols, and mixtures thereof. Also branched chain alcohols obtained in paraffin oxidation or from other sources, such as hydration of olefins or the Reppe process, may be used. Suitable alcohols have been described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1985, volume A1, page 279-303 “Aliphatic alcohols”, VCH Verlagsgesellschaft mbH, Weinheim BRD. Examples of alcohols derived from aldol condensation are 2-ethylhexanol-1, iso-hexadecyl alcohol and iso-octadecyl alcohol. Suitable oxo alcohols are iso-octanol (usually a mixture of about 80% dimethylhexanols, 15% methylheptanols and 5% mixed alcohols), iso-nonanol (about 80% dimethylheptanols and 20% trimethylhexanols), iso-decanol (usually originating from the hydroformylation of tri-propylene), isotridecylalcohol, and the like. Also 2 heptylundecanol, iso-C20 alcohol (such as Isofol-20, ex Condea) octanol-2, and the Guerbet alcohols, such as 2-butyl-octanol-1, 2-nonyl-tridecanol-1, and the like, may be used.
The saturated, branched chain aliphatic monocarboxylic acid having at least 10 carbon atoms may be branched in any position and sometimes branching occurs at several positions in
Appelman Eric
Kenbeek Dirk
Rieffe Hendrik L.
Medley Margaret
Toomer Cephia D.
Unilever Patent Holdings B.V.
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